diff --git a/src/info.cpp b/src/info.cpp
index bbc94fbccb..61781c9b3e 100644
--- a/src/info.cpp
+++ b/src/info.cpp
@@ -566,16 +566,13 @@ void Info::command(int narg, char **arg)
   }
 
   if (flags & COMPUTES) {
-    int ncompute = modify->ncompute;
-    Compute **compute = modify->compute;
+    int i = 0;
     char **names = group->names;
     fputs("\nCompute information:\n",out);
-    for (int i=0; i < ncompute; ++i) {
-      fmt::print(out,"Compute[{:3d}]:  {:16}  style = {:16}  group = {}\n",
-                 i, std::string(compute[i]->id)+',',
-                 std::string(compute[i]->style)+',',
-                 names[compute[i]->igroup]);
-    }
+    for (auto compute : modify->get_compute_list())
+      fmt::print(out,"Compute[{:3d}]:  {:16}  style = {:16}  group = {}\n", i++,
+                 std::string(compute->id)+',',std::string(compute->style)+',',
+                 names[compute->igroup]);
   }
 
   if (flags & DUMPS) {
@@ -587,10 +584,8 @@ void Info::command(int narg, char **arg)
     fputs("\nDump information:\n",out);
     for (int i=0; i < ndump; ++i) {
       fmt::print(out,"Dump[{:3d}]:     {:16}  file = {:16}  style = {:16}  group = {:16}  ",
-                 i, std::string(dump[i]->id)+',',
-                 std::string(dump[i]->filename)+',',
-                 std::string(dump[i]->style)+',',
-                 std::string(names[dump[i]->igroup])+',');
+                 i, std::string(dump[i]->id)+',',std::string(dump[i]->filename)+',',
+                 std::string(dump[i]->style)+',',std::string(names[dump[i]->igroup])+',');
       if (nevery[i]) {
         fmt::print(out,"every = {}\n", nevery[i]);
       } else {
@@ -600,16 +595,12 @@ void Info::command(int narg, char **arg)
   }
 
   if (flags & FIXES) {
-    int nfix = modify->nfix;
-    Fix **fix = modify->fix;
+    int i = 0;
     char **names = group->names;
     fputs("\nFix information:\n",out);
-    for (int i=0; i < nfix; ++i) {
-      fmt::print(out, "Fix[{:3d}]:      {:16}  style = {:16}  group = {}\n",
-                 i,std::string(fix[i]->id)+',',
-                 std::string(fix[i]->style)+',',
-                 names[fix[i]->igroup]);
-    }
+    for (auto fix : modify->get_fix_list())
+      fmt::print(out, "Fix[{:3d}]:      {:16}  style = {:16}  group = {}\n",i++,
+                 std::string(fix->id)+',',std::string(fix->style)+',',names[fix->igroup]);
   }
 
   if (flags & VARIABLES) {
@@ -621,8 +612,7 @@ void Info::command(int narg, char **arg)
     for (int i=0; i < nvar; ++i) {
       int ndata = 1;
       fmt::print(out,"Variable[{:3d}]: {:16}  style = {:16}  def =",
-                 i,std::string(names[i])+',',
-                 std::string(varstyles[style[i]])+',');
+                 i,std::string(names[i])+',',std::string(varstyles[style[i]])+',');
       if (style[i] == Variable::INTERNAL) {
         fmt::print(out,"{:.8}\n",input->variable->dvalue[i]);
         continue;
@@ -797,13 +787,13 @@ bool Info::is_active(const char *category, const char *name)
 
   if (strcmp(category,"package") == 0) {
     if (strcmp(name,"gpu") == 0) {
-      return (modify->find_fix("package_gpu") >= 0) ? true : false;
+      return (modify->get_fix_by_id("package_gpu")) ? true : false;
     } else if (strcmp(name,"intel") == 0) {
-      return (modify->find_fix("package_intel") >= 0) ? true : false;
+      return (modify->get_fix_by_id("package_intel")) ? true : false;
     } else if (strcmp(name,"kokkos") == 0) {
       return (lmp->kokkos && lmp->kokkos->kokkos_exists) ? true : false;
     } else if (strcmp(name,"omp") == 0) {
-      return (modify->find_fix("package_omp") >= 0) ? true : false;
+      return (modify->get_fix_by_id("package_omp")) ? true : false;
     } else error->all(FLERR,"Unknown name for info package category: {}", name);
 
   } else if (strcmp(category,"newton") == 0) {
diff --git a/src/library.cpp b/src/library.cpp
index ed208413e6..a27da0d478 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -1689,9 +1689,8 @@ void *lammps_extract_compute(void *handle, const char *id, int style, int type)
 
   BEGIN_CAPTURE
   {
-    int icompute = lmp->modify->find_compute(id);
-    if (icompute < 0) return nullptr;
-    Compute *compute = lmp->modify->compute[icompute];
+    auto compute = lmp->modify->get_compute_by_id(id);
+    if (!compute) return nullptr;
 
     if (style == LMP_STYLE_GLOBAL) {
       if (type == LMP_TYPE_SCALAR) {
@@ -1876,9 +1875,8 @@ void *lammps_extract_fix(void *handle, const char *id, int style, int type,
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0) return nullptr;
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) return nullptr;
 
     if (style == LMP_STYLE_GLOBAL) {
       if (type == LMP_TYPE_SCALAR) {
@@ -2868,10 +2866,9 @@ void lammps_gather(void *handle, char *name, int type, int count, void *data)
   BEGIN_CAPTURE
   {
 #if defined(LAMMPS_BIGBIG)
-    lmp->error->all(FLERR,"Library function lammps_gather"
-                    " not compatible with -DLAMMPS_BIGBIG");
+    lmp->error->all(FLERR,"Library function lammps_gather not compatible with -DLAMMPS_BIGBIG");
 #else
-    int i,j,offset,fcid,ltype,icol;
+    int i,j,offset,ltype;
 
     // error if tags are not defined or not consecutive
 
@@ -2892,76 +2889,72 @@ void lammps_gather(void *handle, char *name, int type, int count, void *data)
 
     if (vptr==nullptr && utils::strmatch(name,"^f_")) {
 
-      fcid = lmp->modify->find_fix(&name[2]);
-      if (fcid < 0) {
+      auto fix = lmp->modify->get_fix_by_id(&name[2]);
+      if (!fix) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather: unknown fix id");
         return;
       }
 
-      if (lmp->modify->fix[fcid]->peratom_flag == 0) {
+      if (fix->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_gather:"
-                              " fix does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_gather: fix does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->fix[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (fix->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_gather:"
-                              " count != values peratom for fix");
+          lmp->error->warning(FLERR,"lammps_gather: count != values peratom for fix");
         return;
       }
 
-      if (lmp->update->ntimestep % lmp->modify->fix[fcid]->peratom_freq) {
+      if (lmp->update->ntimestep % fix->peratom_freq) {
         if (lmp->comm->me == 0)
-          lmp->error->all(FLERR,"lammps_gather:"
-                          " fix not computed at compatible time");
+          lmp->error->all(FLERR,"lammps_gather: fix not computed at compatible time");
         return;
       }
 
-      if (count==1) vptr = (void *) lmp->modify->fix[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->fix[fcid]->array_atom;
+      if (count==1) vptr = (void *) fix->vector_atom;
+      else vptr = (void *) fix->array_atom;
     }
 
     // compute
 
     if (vptr==nullptr && utils::strmatch(name,"^c_")) {
 
-      fcid = lmp->modify->find_compute(&name[2]);
-      if (fcid < 0) {
+      auto compute = lmp->modify->get_compute_by_id(&name[2]);
+      if (!compute) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather: unknown compute id");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->peratom_flag == 0) {
+      if (compute->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_gather:"
-                              " compute does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_gather: compute does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->compute[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (compute->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_gather:"
-                              " count != values peratom for compute");
+          lmp->error->warning(FLERR,"lammps_gather: count != values peratom for compute");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->invoked_peratom != lmp->update->ntimestep)
-        lmp->modify->compute[fcid]->compute_peratom();
+      if (compute->invoked_peratom != lmp->update->ntimestep)
+        compute->compute_peratom();
 
-      if (count==1) vptr = (void *) lmp->modify->compute[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->compute[fcid]->array_atom;
+      if (count==1) vptr = (void *) compute->vector_atom;
+      else vptr = (void *) compute->array_atom;
     }
 
     // custom fix property/atom vector or array
 
     if ((vptr == nullptr) && utils::strmatch(name,"^[id]2?_")) {
 
-      if (utils::strmatch(name,"^[id]_")) fcid = lmp->atom->find_custom(&name[2],ltype,icol);
-      else fcid = lmp->atom->find_custom(&name[3],ltype,icol);
+      int idx,icol;
+      if (utils::strmatch(name,"^[id]_")) idx = lmp->atom->find_custom(&name[2],ltype,icol);
+      else idx = lmp->atom->find_custom(&name[3],ltype,icol);
 
-      if (fcid < 0) {
+      if (idx < 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather: unknown property/atom id");
         return;
@@ -2972,23 +2965,23 @@ void lammps_gather(void *handle, char *name, int type, int count, void *data)
           lmp->error->warning(FLERR,"lammps_gather: mismatch property/atom type");
         return;
       }
-      if (count == 1 && icol != 0) {
+      if ((count == 1) && (icol != 0)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather: mismatch property/atom count");
         return;
       }
-      if (count > 1 && icol != count) {
+      if ((count > 1) && (icol != count)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather: mismatch property/atom count");
         return;
       }
 
       if (count == 1) {
-        if (ltype==0) vptr = (void *) lmp->atom->ivector[fcid];
-        else vptr = (void *) lmp->atom->dvector[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->ivector[idx];
+        else vptr = (void *) lmp->atom->dvector[idx];
       } else {
-        if (ltype==0) vptr = (void *) lmp->atom->iarray[fcid];
-        else vptr = (void *) lmp->atom->darray[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->iarray[idx];
+        else vptr = (void *) lmp->atom->darray[idx];
       }
     }
 
@@ -3111,7 +3104,7 @@ void lammps_gather_concat(void *handle, char *name, int type, int count, void *d
     lmp->error->all(FLERR,"Library function lammps_gather_concat"
                           " not compatible with -DLAMMPS_BIGBIG");
 #else
-    int i,offset,fcid,ltype,icol;
+    int i,offset,ltype;
 
     // error if tags are not defined or not consecutive
 
@@ -3131,70 +3124,71 @@ void lammps_gather_concat(void *handle, char *name, int type, int count, void *d
 
     if (vptr==nullptr && utils::strmatch(name,"^f_")) {
 
-      fcid = lmp->modify->find_fix(&name[2]);
-      if (fcid < 0) {
+      auto fix = lmp->modify->get_fix_by_id(&name[2]);
+      if (!fix) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: unknown fix id");
         return;
       }
 
-      if (lmp->modify->fix[fcid]->peratom_flag == 0) {
+      if (fix->peratom_flag == 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: fix does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->fix[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (fix->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: count != values peratom for fix");
         return;
       }
-      if (lmp->update->ntimestep % lmp->modify->fix[fcid]->peratom_freq) {
+      if (lmp->update->ntimestep % fix->peratom_freq) {
         if (lmp->comm->me == 0)
           lmp->error->all(FLERR,"lammps_gather_concat: fix not computed at compatible time");
         return;
       }
 
-      if (count==1) vptr = (void *) lmp->modify->fix[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->fix[fcid]->array_atom;
+      if (count==1) vptr = (void *) fix->vector_atom;
+      else vptr = (void *) fix->array_atom;
     }
 
     // compute
 
     if (vptr==nullptr && utils::strmatch(name,"^c_")) {
 
-      fcid = lmp->modify->find_compute(&name[2]);
-      if (fcid < 0) {
+      auto compute = lmp->modify->get_compute_by_id(&name[2]);
+      if (!compute) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: unknown compute id");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->peratom_flag == 0) {
+      if (compute->peratom_flag == 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: compute does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->compute[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (compute->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: count != values peratom for compute");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->invoked_peratom != lmp->update->ntimestep)
-        lmp->modify->compute[fcid]->compute_peratom();
+      if (compute->invoked_peratom != lmp->update->ntimestep)
+        compute->compute_peratom();
 
-      if (count==1) vptr = (void *) lmp->modify->compute[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->compute[fcid]->array_atom;
+      if (count==1) vptr = (void *) compute->vector_atom;
+      else vptr = (void *) compute->array_atom;
     }
 
     // custom per-atom vector or array
 
     if ((vptr==nullptr) && utils::strmatch(name,"^[id]2?_")) {
 
-      if (utils::strmatch(name,"^[id]_")) fcid = lmp->atom->find_custom(&name[2],ltype,icol);
-      else fcid = lmp->atom->find_custom(&name[3],ltype,icol);
+      int idx,icol;
+      if (utils::strmatch(name,"^[id]_")) idx = lmp->atom->find_custom(&name[2],ltype,icol);
+      else idx = lmp->atom->find_custom(&name[3],ltype,icol);
 
-      if (fcid < 0) {
+      if (idx < 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: unknown property/atom id");
         return;
@@ -3205,23 +3199,23 @@ void lammps_gather_concat(void *handle, char *name, int type, int count, void *d
           lmp->error->warning(FLERR,"lammps_gather_concat: mismatch property/atom type");
         return;
       }
-      if (count == 1 && icol != 0) {
+      if ((count == 1) && (icol != 0)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: mismatch property/atom count");
         return;
       }
-      if (count > 1 && icol != count) {
+      if ((count > 1) && (icol != count)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_concat: mismatch property/atom count");
         return;
       }
 
       if (count == 1) {
-        if (ltype==0) vptr = (void *) lmp->atom->ivector[fcid];
-        else vptr = (void *) lmp->atom->dvector[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->ivector[idx];
+        else vptr = (void *) lmp->atom->dvector[idx];
       } else {
-        if (ltype==0) vptr = (void *) lmp->atom->iarray[fcid];
-        else vptr = (void *) lmp->atom->darray[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->iarray[idx];
+        else vptr = (void *) lmp->atom->darray[idx];
       }
     }
 
@@ -3364,7 +3358,7 @@ void lammps_gather_subset(void *handle, char *name,
     lmp->error->all(FLERR,"Library function lammps_gather_subset() "
                     "is not compatible with -DLAMMPS_BIGBIG");
 #else
-    int i,j,m,offset,fcid,ltype,icol;
+    int i,j,m,offset,ltype;
     tagint id;
 
     // error if tags are not defined or not consecutive
@@ -3384,71 +3378,70 @@ void lammps_gather_subset(void *handle, char *name,
 
     if (vptr==nullptr && utils::strmatch(name,"^f_")) {
 
-      fcid = lmp->modify->find_fix(&name[2]);
-      if (fcid < 0) {
+      auto fix = lmp->modify->get_fix_by_id(&name[2]);
+      if (!fix) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_subset: unknown fix id");
         return;
       }
 
-      if (lmp->modify->fix[fcid]->peratom_flag == 0) {
+      if (fix->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_gather_subset:"
-                              " fix does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_gather_subset: fix does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->fix[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (fix->size_peratom_cols != count)) {
         lmp->error->warning(FLERR,"lammps_gather_subset: count != values peratom for fix");
         return;
       }
-      if (lmp->update->ntimestep % lmp->modify->fix[fcid]->peratom_freq) {
+      if (lmp->update->ntimestep % fix->peratom_freq) {
         if (lmp->comm->me == 0)
           lmp->error->all(FLERR,"lammps_gather_subset: fix not computed at compatible time");
         return;
       }
 
-      if (count==1) vptr = (void *) lmp->modify->fix[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->fix[fcid]->array_atom;
+      if (count==1) vptr = (void *) fix->vector_atom;
+      else vptr = (void *) fix->array_atom;
     }
 
     // compute
 
     if (vptr==nullptr && utils::strmatch(name,"^c_")) {
 
-      fcid = lmp->modify->find_compute(&name[2]);
-      if (fcid < 0) {
+      auto compute = lmp->modify->get_compute_by_id(&name[2]);
+      if (!compute) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_subset: unknown compute id");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->peratom_flag == 0) {
+      if (compute->peratom_flag == 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_subset: compute does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->compute[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (compute->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_subset: count != values peratom for compute");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->invoked_peratom != lmp->update->ntimestep)
-        lmp->modify->compute[fcid]->compute_peratom();
+      if (compute->invoked_peratom != lmp->update->ntimestep)
+        compute->compute_peratom();
 
-      if (count==1) vptr = (void *) lmp->modify->compute[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->compute[fcid]->array_atom;
+      if (count==1) vptr = (void *) compute->vector_atom;
+      else vptr = (void *) compute->array_atom;
     }
 
     // custom fix property/atom vector or array
 
     if ((vptr == nullptr) && utils::strmatch(name,"^[id]2?_")) {
 
-      if (utils::strmatch(name,"^[id]_"))
-        fcid = lmp->atom->find_custom(&name[2],ltype,icol);
-      else fcid = lmp->atom->find_custom(&name[3],ltype,icol);
+      int idx,icol;
+      if (utils::strmatch(name,"^[id]_")) idx = lmp->atom->find_custom(&name[2],ltype,icol);
+      else idx = lmp->atom->find_custom(&name[3],ltype,icol);
 
-      if (fcid < 0) {
+      if (idx < 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_gather_subset: unknown property/atom id");
         return;
@@ -3471,11 +3464,11 @@ void lammps_gather_subset(void *handle, char *name,
       }
 
       if (count == 1) {
-        if (ltype==0) vptr = (void *) lmp->atom->ivector[fcid];
-        else vptr = (void *) lmp->atom->dvector[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->ivector[idx];
+        else vptr = (void *) lmp->atom->dvector[idx];
       } else {
-        if (ltype==0) vptr = (void *) lmp->atom->iarray[fcid];
-        else vptr = (void *) lmp->atom->darray[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->iarray[idx];
+        else vptr = (void *) lmp->atom->darray[idx];
       }
     }
 
@@ -3611,7 +3604,7 @@ void lammps_scatter(void *handle, char *name, int type, int count, void *data)
     lmp->error->all(FLERR,"Library function lammps_scatter() "
                     "is not compatible with -DLAMMPS_BIGBIG");
 #else
-    int i,j,m,offset,fcid,ltype,icol;
+    int i,j,m,offset,ltype;
 
     // error if tags are not defined or not consecutive or no atom map
     // NOTE: test that name = image or ids is not a 64-bit int in code?
@@ -3634,70 +3627,66 @@ void lammps_scatter(void *handle, char *name, int type, int count, void *data)
 
     if (vptr==nullptr && utils::strmatch(name,"^f_")) {
 
-      fcid = lmp->modify->find_fix(&name[2]);
-      if (fcid < 0) {
+      auto fix = lmp->modify->get_fix_by_id(&name[2]);
+      if (!fix) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter: unknown fix id");
         return;
       }
 
-      if (lmp->modify->fix[fcid]->peratom_flag == 0) {
+      if (fix->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter:"
-                              " fix does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_scatter: fix does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->fix[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (fix->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter:"
-                              " count != values peratom for fix");
+          lmp->error->warning(FLERR,"lammps_scatter: count != values peratom for fix");
         return;
       }
 
-      if (count==1) vptr = (void *) lmp->modify->fix[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->fix[fcid]->array_atom;
+      if (count==1) vptr = (void *) fix->vector_atom;
+      else vptr = (void *) fix->array_atom;
     }
 
     // compute
 
     if (vptr==nullptr && utils::strmatch(name,"^c_")) {
 
-      fcid = lmp->modify->find_compute(&name[2]);
-      if (fcid < 0) {
+      auto compute = lmp->modify->get_compute_by_id(&name[2]);
+      if (!compute) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter: unknown compute id");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->peratom_flag == 0) {
+      if (compute->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter:"
-                              " compute does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_scatter: compute does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->compute[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (compute->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter:"
-                              " count != values peratom for compute");
+          lmp->error->warning(FLERR,"lammps_scatter: count != values peratom for compute");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->invoked_peratom != lmp->update->ntimestep)
-        lmp->modify->compute[fcid]->compute_peratom();
+      if (compute->invoked_peratom != lmp->update->ntimestep)
+        compute->compute_peratom();
 
-      if (count==1) vptr = (void *) lmp->modify->compute[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->compute[fcid]->array_atom;
+      if (count==1) vptr = (void *) compute->vector_atom;
+      else vptr = (void *) compute->array_atom;
     }
 
     // custom fix property/atom vector or array
 
     if ((vptr == nullptr) && utils::strmatch(name,"^[id]2?_")) {
 
-      if (utils::strmatch(name,"^[id]_"))
-        fcid = lmp->atom->find_custom(&name[2],ltype,icol);
-      else fcid = lmp->atom->find_custom(&name[3],ltype,icol);
+      int idx,icol;
+      if (utils::strmatch(name,"^[id]_")) idx = lmp->atom->find_custom(&name[2],ltype,icol);
+      else idx = lmp->atom->find_custom(&name[3],ltype,icol);
 
-      if (fcid < 0) {
+      if (idx < 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter: unknown property/atom id");
         return;
@@ -3720,11 +3709,11 @@ void lammps_scatter(void *handle, char *name, int type, int count, void *data)
       }
 
       if (count == 1) {
-        if (ltype==0) vptr = (void *) lmp->atom->ivector[fcid];
-        else vptr = (void *) lmp->atom->dvector[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->ivector[idx];
+        else vptr = (void *) lmp->atom->dvector[idx];
       } else {
-        if (ltype==0) vptr = (void *) lmp->atom->iarray[fcid];
-        else vptr = (void *) lmp->atom->darray[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->iarray[idx];
+        else vptr = (void *) lmp->atom->darray[idx];
       }
     }
 
@@ -3833,7 +3822,7 @@ void lammps_scatter_subset(void *handle, char *name,int type, int count,
     lmp->error->all(FLERR,"Library function lammps_scatter_subset() "
                     "is not compatible with -DLAMMPS_BIGBIG");
 #else
-    int i,j,m,offset,fcid,ltype,icol;
+    int i,j,m,offset,ltype;
     tagint id;
 
     // error if tags are not defined or no atom map
@@ -3855,70 +3844,66 @@ void lammps_scatter_subset(void *handle, char *name,int type, int count,
 
     if (vptr==nullptr && utils::strmatch(name,"^f_")) {
 
-      fcid = lmp->modify->find_fix(&name[2]);
-      if (fcid < 0) {
+      auto fix = lmp->modify->get_fix_by_id(&name[2]);
+      if (!fix) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter_subset: unknown fix id");
         return;
       }
 
-      if (lmp->modify->fix[fcid]->peratom_flag == 0) {
+      if (fix->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter_subset:"
-                              " fix does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_scatter_subset: fix does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->fix[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (fix->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter_subset:"
-                              " count != values peratom for fix");
+          lmp->error->warning(FLERR,"lammps_scatter_subset: count != values peratom for fix");
         return;
       }
 
-      if (count==1) vptr = (void *) lmp->modify->fix[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->fix[fcid]->array_atom;
+      if (count==1) vptr = (void *) fix->vector_atom;
+      else vptr = (void *) fix->array_atom;
     }
 
     // compute
 
     if (vptr==nullptr && utils::strmatch(name,"^c_")) {
 
-      fcid = lmp->modify->find_compute(&name[2]);
-      if (fcid < 0) {
+      auto compute = lmp->modify->get_compute_by_id(&name[2]);
+      if (!compute) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter_subset: unknown compute id");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->peratom_flag == 0) {
+      if (compute->peratom_flag == 0) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter_subset:"
-                              " compute does not return peratom data");
+          lmp->error->warning(FLERR,"lammps_scatter_subset: compute does not return peratom data");
         return;
       }
-      if (count>1 && lmp->modify->compute[fcid]->size_peratom_cols != count) {
+      if ((count > 1) && (compute->size_peratom_cols != count)) {
         if (lmp->comm->me == 0)
-          lmp->error->warning(FLERR,"lammps_scatter_subset:"
-                              " count != values peratom for compute");
+          lmp->error->warning(FLERR,"lammps_scatter_subset: count != values peratom for compute");
         return;
       }
 
-      if (lmp->modify->compute[fcid]->invoked_peratom != lmp->update->ntimestep)
-        lmp->modify->compute[fcid]->compute_peratom();
+      if (compute->invoked_peratom != lmp->update->ntimestep)
+        compute->compute_peratom();
 
-      if (count==1) vptr = (void *) lmp->modify->compute[fcid]->vector_atom;
-      else vptr = (void *) lmp->modify->compute[fcid]->array_atom;
+      if (count==1) vptr = (void *) compute->vector_atom;
+      else vptr = (void *) compute->array_atom;
     }
 
     // custom fix property/atom vector or array
 
     if ((vptr == nullptr) && utils::strmatch(name,"^[id]2?_")) {
 
-      if (utils::strmatch(name,"^[id]_"))
-        fcid = lmp->atom->find_custom(&name[2],ltype,icol);
-      else fcid = lmp->atom->find_custom(&name[3],ltype,icol);
+      int idx,icol;
+      if (utils::strmatch(name,"^[id]_")) idx = lmp->atom->find_custom(&name[2],ltype,icol);
+      else idx = lmp->atom->find_custom(&name[3],ltype,icol);
 
-      if (fcid < 0) {
+      if (idx < 0) {
         if (lmp->comm->me == 0)
           lmp->error->warning(FLERR,"lammps_scatter_subset: unknown property/atom id");
         return;
@@ -3941,11 +3926,11 @@ void lammps_scatter_subset(void *handle, char *name,int type, int count,
       }
 
       if (count == 1) {
-        if (ltype==0) vptr = (void *) lmp->atom->ivector[fcid];
-        else vptr = (void *) lmp->atom->dvector[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->ivector[idx];
+        else vptr = (void *) lmp->atom->dvector[idx];
       } else {
-        if (ltype==0) vptr = (void *) lmp->atom->iarray[fcid];
-        else vptr = (void *) lmp->atom->darray[fcid];
+        if (ltype==0) vptr = (void *) lmp->atom->iarray[idx];
+        else vptr = (void *) lmp->atom->darray[idx];
       }
     }
 
@@ -4242,10 +4227,9 @@ int lammps_find_pair_neighlist(void *handle, const char *style, int exact, int n
 
 int lammps_find_fix_neighlist(void *handle, const char *id, int reqid) {
   LAMMPS *lmp = (LAMMPS *) handle;
-  const int ifix = lmp->modify->find_fix(id);
-  if (ifix < 0) return -1;
+  auto fix = lmp->modify->get_fix_by_id(id);
+  if (!fix) return -1;
 
-  Fix *fix = lmp->modify->fix[ifix];
   // find neigh list
   for (int i = 0; i < lmp->neighbor->nlist; i++) {
     NeighList *list = lmp->neighbor->lists[i];
@@ -4272,10 +4256,9 @@ int lammps_find_fix_neighlist(void *handle, const char *id, int reqid) {
 
 int lammps_find_compute_neighlist(void* handle, const char *id, int reqid) {
   LAMMPS *lmp = (LAMMPS *) handle;
-  const int icompute = lmp->modify->find_compute(id);
-  if (icompute < 0) return -1;
+  auto compute = lmp->modify->get_compute_by_id(id);
+  if (!compute) return -1;
 
-  Compute *compute = lmp->modify->compute[icompute];
   // find neigh list
   for (int i = 0; i < lmp->neighbor->nlist; i++) {
     NeighList * list = lmp->neighbor->lists[i];
@@ -5089,11 +5072,8 @@ void lammps_set_fix_external_callback(void *handle, const char *id, FixExternalF
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Cannot find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Cannot find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style 'external'", id);
@@ -5153,16 +5133,14 @@ double **lammps_fix_external_get_force(void *handle, const char *id)
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
 
-    fexternal = (double **)fix->extract("fexternal",ifix);
+    int tmp;
+    fexternal = (double **)fix->extract("fexternal",tmp);
   }
   END_CAPTURE
   return fexternal;
@@ -5202,11 +5180,8 @@ void lammps_fix_external_set_energy_global(void *handle, const char *id, double
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
@@ -5253,11 +5228,8 @@ void lammps_fix_external_set_virial_global(void *handle, const char *id, double
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
@@ -5304,11 +5276,8 @@ void lammps_fix_external_set_energy_peratom(void *handle, const char *id, double
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
@@ -5358,11 +5327,8 @@ void lammps_fix_external_set_virial_peratom(void *handle, const char *id, double
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
@@ -5405,11 +5371,8 @@ void lammps_fix_external_set_vector_length(void *handle, const char *id, int len
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
@@ -5462,11 +5425,8 @@ void lammps_fix_external_set_vector(void *handle, const char *id, int idx, doubl
 
   BEGIN_CAPTURE
   {
-    int ifix = lmp->modify->find_fix(id);
-    if (ifix < 0)
-      lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
-
-    Fix *fix = lmp->modify->fix[ifix];
+    auto fix = lmp->modify->get_fix_by_id(id);
+    if (!fix) lmp->error->all(FLERR,"Can not find fix with ID '{}'!", id);
 
     if (strcmp("external",fix->style) != 0)
       lmp->error->all(FLERR,"Fix '{}' is not of style external!", id);
diff --git a/src/min.cpp b/src/min.cpp
index 4780cf31eb..0c71607bd8 100644
--- a/src/min.cpp
+++ b/src/min.cpp
@@ -157,8 +157,7 @@ void Min::init()
 
   // detect if fix omp is present for clearing force arrays
 
-  int ifix = modify->find_fix("package_omp");
-  if (ifix >= 0) external_force_clear = 1;
+  if (modify->get_fix_by_id("package_omp")) external_force_clear = 1;
 
   // set flags for arrays to clear in force_clear()
 
@@ -230,9 +229,8 @@ void Min::setup(int flag)
 
   // compute for potential energy
 
-  int id = modify->find_compute("thermo_pe");
-  if (id < 0) error->all(FLERR,"Minimization could not find thermo_pe compute");
-  pe_compute = modify->compute[id];
+  pe_compute = modify->get_compute_by_id("thermo_pe");
+  if (!pe_compute) error->all(FLERR,"Minimization could not find thermo_pe compute");
 
   // style-specific setup does two tasks
   // setup extra global dof vectors
diff --git a/src/modify.cpp b/src/modify.cpp
index aadfd5cc3e..1ff4986a43 100644
--- a/src/modify.cpp
+++ b/src/modify.cpp
@@ -807,9 +807,8 @@ Fix *Modify::add_fix(int narg, char **arg, int trysuffix)
   //   since some fixes access domain settings in their constructor
   // nullptr must be last entry in this list
 
-  const char *exceptions[] =
-    {"GPU", "OMP", "INTEL", "property/atom", "cmap", "cmap3", "rx",
-     "deprecated", "STORE/KIM", nullptr};
+  const char *exceptions[] = {"GPU", "OMP", "INTEL", "property/atom", "cmap", "cmap3", "rx",
+    "deprecated", "STORE/KIM", nullptr};
 
   if (domain->box_exist == 0) {
     int m;
@@ -822,7 +821,7 @@ Fix *Modify::add_fix(int narg, char **arg, int trysuffix)
   // check group ID
 
   int igroup = group->find(arg[1]);
-  if (igroup == -1) error->all(FLERR,"Could not find fix group ID");
+  if (igroup == -1) error->all(FLERR,"Could not find fix group ID {}", arg[1]);
 
   // if fix ID exists:
   //   set newflag = 0 so create new fix in same location in fix list
@@ -856,8 +855,7 @@ Fix *Modify::add_fix(int narg, char **arg, int trysuffix)
         if (estyle == fix[ifix]->style) match = 1;
       }
     }
-    if (!match)
-      error->all(FLERR,"Replacing a fix, but new style != old style");
+    if (!match) error->all(FLERR,"Replacing a fix, but new style != old style");
 
     if (fix[ifix]->igroup != igroup && comm->me == 0)
       error->warning(FLERR,"Replacing a fix, but new group != old group");
@@ -907,9 +905,12 @@ Fix *Modify::add_fix(int narg, char **arg, int trysuffix)
   if (fix[ifix] == nullptr)
     error->all(FLERR,utils::check_packages_for_style("fix",arg[2],lmp));
 
-  // increment nfix (if new)
+  // increment nfix and update fix_list vector (if new)
 
-  if (newflag) nfix++;
+  if (newflag) {
+    nfix++;
+    fix_list = std::vector<Fix *>(fix, fix+nfix);
+  }
 
   // post_constructor() can call virtual methods in parent or child
   //   which would otherwise not yet be visible in child class
@@ -983,25 +984,24 @@ Fix *Modify::add_fix(const std::string &fixcmd, int trysuffix)
 
 Fix *Modify::replace_fix(const char *replaceID, int narg, char **arg, int trysuffix)
 {
-  int ifix = find_fix(replaceID);
-  if (ifix < 0) error->all(FLERR,"Modify replace_fix ID {} could not be found", replaceID);
+  auto oldfix = get_fix_by_id(replaceID);
+  if (!oldfix) error->all(FLERR,"Modify replace_fix ID {} could not be found", replaceID);
 
   // change ID, igroup, style of fix being replaced to match new fix
   // requires some error checking on arguments for new fix
 
   if (narg < 3) error->all(FLERR,"Illegal replace_fix invocation");
-  int jfix = find_fix(arg[0]);
-  if (jfix >= 0) error->all(FLERR,"Replace_fix ID is already in use");
+  if (!get_fix_by_id(arg[0])) error->all(FLERR,"Replace_fix ID is already in use");
 
-  delete [] fix[ifix]->id;
-  fix[ifix]->id = utils::strdup(arg[0]);
+  delete[] oldfix->id;
+  oldfix->id = utils::strdup(arg[0]);
 
   int jgroup = group->find(arg[1]);
-  if (jgroup == -1) error->all(FLERR,"Could not find replace_fix group ID");
-  fix[ifix]->igroup = jgroup;
+  if (jgroup == -1) error->all(FLERR,"Could not find replace_fix group ID {}", arg[1]);
+  oldfix->igroup = jgroup;
 
-  delete [] fix[ifix]->style;
-  fix[ifix]->style = utils::strdup(arg[2]);
+  delete[] oldfix->style;
+  oldfix->style = utils::strdup(arg[2]);
 
   // invoke add_fix
   // it will find and overwrite the replaceID fix
@@ -1047,7 +1047,7 @@ void Modify::modify_fix(int narg, char **arg)
   int ifix;
   for (ifix = 0; ifix < nfix; ifix++)
     if (strcmp(arg[0],fix[ifix]->id) == 0) break;
-  if (ifix == nfix) error->all(FLERR,"Could not find fix_modify ID");
+  if (ifix == nfix) error->all(FLERR,"Could not find fix_modify ID {}", arg[0]);
 
   fix[ifix]->modify_params(narg-1,&arg[1]);
 }
@@ -1060,7 +1060,7 @@ void Modify::modify_fix(int narg, char **arg)
 void Modify::delete_fix(const std::string &id)
 {
   int ifix = find_fix(id);
-  if (ifix < 0) error->all(FLERR,"Could not find fix ID to delete");
+  if (ifix < 0) error->all(FLERR,"Could not find fix ID {} to delete", id);
   delete_fix(ifix);
 }
 
@@ -1076,6 +1076,7 @@ void Modify::delete_fix(int ifix)
   for (int i = ifix+1; i < nfix; i++) fix[i-1] = fix[i];
   for (int i = ifix+1; i < nfix; i++) fmask[i-1] = fmask[i];
   nfix--;
+  fix_list = std::vector<Fix *>(fix, fix+nfix);
 }
 
 /* ----------------------------------------------------------------------
@@ -1252,8 +1253,7 @@ Compute *Modify::add_compute(int narg, char **arg, int trysuffix)
 
   if (ncompute == maxcompute) {
     maxcompute += DELTA;
-    compute = (Compute **)
-      memory->srealloc(compute,maxcompute*sizeof(Compute *),"modify:compute");
+    compute = (Compute **) memory->srealloc(compute,maxcompute*sizeof(Compute *),"modify:compute");
   }
 
   // create the Compute
@@ -1291,6 +1291,7 @@ Compute *Modify::add_compute(int narg, char **arg, int trysuffix)
   if (compute[ncompute] == nullptr)
     error->all(FLERR,utils::check_packages_for_style("compute",arg[2],lmp));
 
+  compute_list = std::vector<Compute *>(compute, compute+ncompute+1);
   return compute[ncompute++];
 }
 
@@ -1334,7 +1335,7 @@ void Modify::modify_compute(int narg, char **arg)
   for (icompute = 0; icompute < ncompute; icompute++)
     if (strcmp(arg[0],compute[icompute]->id) == 0) break;
   if (icompute == ncompute)
-    error->all(FLERR,"Could not find compute_modify ID");
+    error->all(FLERR,"Could not find compute_modify ID {}", arg[0]);
 
   compute[icompute]->modify_params(narg-1,&arg[1]);
 }
@@ -1346,7 +1347,7 @@ void Modify::modify_compute(int narg, char **arg)
 void Modify::delete_compute(const std::string &id)
 {
   int icompute = find_compute(id);
-  if (icompute < 0) error->all(FLERR,"Could not find compute ID to delete");
+  if (icompute < 0) error->all(FLERR,"Could not find compute ID {} to delete", id);
   delete_compute(icompute);
 }
 
@@ -1359,6 +1360,7 @@ void Modify::delete_compute(int icompute)
   delete compute[icompute];
   for (int i = icompute+1; i < ncompute; i++) compute[i-1] = compute[i];
   ncompute--;
+  compute_list = std::vector<Compute *>(compute, compute+ncompute);
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/read_data.cpp b/src/read_data.cpp
index edbfc16696..27d60275c4 100644
--- a/src/read_data.cpp
+++ b/src/read_data.cpp
@@ -267,14 +267,11 @@ void ReadData::command(int narg, char **arg)
         error->all(FLERR,"Illegal read_data command");
       memory->grow(fix_index,nfix+1,"read_data:fix_index");
       fix_header = (char **)
-        memory->srealloc(fix_header,(nfix+1)*sizeof(char *),
-                         "read_data:fix_header");
+        memory->srealloc(fix_header,(nfix+1)*sizeof(char *),"read_data:fix_header");
       fix_section = (char **)
-        memory->srealloc(fix_section,(nfix+1)*sizeof(char *),
-                         "read_data:fix_section");
+        memory->srealloc(fix_section,(nfix+1)*sizeof(char *),"read_data:fix_section");
       fix_index[nfix] = modify->find_fix(arg[iarg+1]);
-      if (fix_index[nfix] < 0)
-        error->all(FLERR,"Fix ID for read_data does not exist");
+      if (fix_index[nfix] < 0) error->all(FLERR,"Fix ID for read_data does not exist");
       if (strcmp(arg[iarg+2],"NULL") == 0) fix_header[nfix] = nullptr;
       else fix_header[nfix] = utils::strdup(arg[iarg+2]);
       fix_section[nfix] = utils::strdup(arg[iarg+3]);
diff --git a/src/read_restart.cpp b/src/read_restart.cpp
index 0eab44baaa..f8ac14534b 100644
--- a/src/read_restart.cpp
+++ b/src/read_restart.cpp
@@ -445,8 +445,7 @@ void ReadRestart::command(int narg, char **arg)
     if (nextra) {
       memory->destroy(atom->extra);
       memory->create(atom->extra,atom->nmax,nextra,"atom:extra");
-      int ifix = modify->find_fix("_read_restart");
-      FixReadRestart *fix = (FixReadRestart *) modify->fix[ifix];
+      auto fix = (FixReadRestart *) modify->get_fix_by_id("_read_restart");
       int *count = fix->count;
       double **extra = fix->extra;
       double **atom_extra = atom->extra;
diff --git a/src/reset_mol_ids.cpp b/src/reset_mol_ids.cpp
index 4c973f543f..a29ea98e18 100644
--- a/src/reset_mol_ids.cpp
+++ b/src/reset_mol_ids.cpp
@@ -151,24 +151,14 @@ void ResetMolIDs::create_computes(char *fixid, char *groupid)
   // 'fixid' allows for creating independent instances of the computes
 
   idfrag = fmt::format("{}_reset_mol_ids_FRAGMENT_ATOM",fixid);
-  if (singleflag)
-    modify->add_compute(fmt::format("{} {} fragment/atom single yes",idfrag,groupid));
-  else
-    modify->add_compute(fmt::format("{} {} fragment/atom single no",idfrag,groupid));
+  auto use_single = singleflag ? "yes" : "no";
+  cfa = (ComputeFragmentAtom *)
+    modify->add_compute(fmt::format("{} {} fragment/atom single {}",idfrag,groupid,use_single));
 
   idchunk = fmt::format("{}_reset_mol_ids_CHUNK_ATOM",fixid);
   if (compressflag)
-    modify->add_compute(fmt::format("{} {} chunk/atom molecule compress yes",
-                                    idchunk,groupid));
-
-  int icompute = modify->find_compute(idfrag);
-  cfa = (ComputeFragmentAtom *) modify->compute[icompute];
-
-
-  if (compressflag) {
-    icompute = modify->find_compute(idchunk);
-    cca = (ComputeChunkAtom *) modify->compute[icompute];
-  }
+    cca = (ComputeChunkAtom *)
+      modify->add_compute(fmt::format("{} {} chunk/atom molecule compress yes",idchunk,groupid));
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/respa.cpp b/src/respa.cpp
index 5190441994..cc8c03538d 100644
--- a/src/respa.cpp
+++ b/src/respa.cpp
@@ -322,8 +322,7 @@ void Respa::init()
 
   // detect if fix omp is present and will clear force arrays
 
-  int ifix = modify->find_fix("package_omp");
-  if (ifix >= 0) external_force_clear = 1;
+  if (modify->get_fix_by_id("package_omp")) external_force_clear = 1;
 
   // set flags for arrays to clear in force_clear()
 
diff --git a/src/thermo.cpp b/src/thermo.cpp
index 434012870b..e39d7d7c57 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -267,23 +267,20 @@ void Thermo::init()
 
   // find current ptr for each Compute ID
 
-  int icompute;
   for (i = 0; i < ncompute; i++) {
-    icompute = modify->find_compute(id_compute[i]);
-    if (icompute < 0) error->all(FLERR,"Could not find thermo compute ID");
-    computes[i] = modify->compute[icompute];
+    computes[i] = modify->get_compute_by_id(id_compute[i]);
+    if (!computes[i]) error->all(FLERR,"Could not find thermo compute with ID {}",id_compute[i]);
   }
 
   // find current ptr for each Fix ID
   // check that fix frequency is acceptable with thermo output frequency
 
-  int ifix;
   for (i = 0; i < nfix; i++) {
-    ifix = modify->find_fix(id_fix[i]);
-    if (ifix < 0) error->all(FLERR,"Could not find thermo fix ID");
-    fixes[i] = modify->fix[ifix];
+    fixes[i] = modify->get_fix_by_id(id_fix[i]);
+    if (!fixes[i]) error->all(FLERR,"Could not find thermo fix ID {}",id_fix[i]);
+
     if (output->thermo_every % fixes[i]->global_freq)
-      error->all(FLERR,"Thermo and fix not computed at compatible times");
+      error->all(FLERR,"Thermo and fix {} not computed at compatible times",id_fix[i]);
   }
 
   // find current ptr for each Variable ID
@@ -467,36 +464,33 @@ void Thermo::modify_params(int narg, char **arg)
     if (strcmp(arg[iarg],"temp") == 0) {
       if (iarg+2 > narg) error->all(FLERR,"Illegal thermo_modify command");
       if (index_temp < 0) error->all(FLERR,"Thermo style does not use temp");
-      delete [] id_compute[index_temp];
+      delete[] id_compute[index_temp];
       id_compute[index_temp] = utils::strdup(arg[iarg+1]);
 
-      int icompute = modify->find_compute(arg[iarg+1]);
-      if (icompute < 0)
-        error->all(FLERR,"Could not find thermo_modify temperature ID");
-      temperature = modify->compute[icompute];
+      temperature = modify->get_compute_by_id(arg[iarg+1]);
+      if (!temperature)
+        error->all(FLERR,"Could not find thermo_modify temperature compute {}",arg[iarg+1]);
 
       if (temperature->tempflag == 0)
-        error->all(FLERR,"Thermo_modify temperature ID does not "
-                   "compute temperature");
+        error->all(FLERR,"Thermo_modify compute {} does not compute temperature",arg[iarg+1]);
       if (temperature->igroup != 0 && comm->me == 0)
-        error->warning(FLERR,
-                       "Temperature for thermo pressure is not for group all");
+        error->warning(FLERR,"Temperature for thermo pressure is not for group all");
 
       // reset id_temp of pressure to new temperature ID
       // either pressure currently being used by thermo or "thermo_press"
 
+      Compute *pcompute;
       if (index_press_scalar >= 0) {
-        icompute = modify->find_compute(id_compute[index_press_scalar]);
-        if (icompute < 0) error->all(FLERR,
-                                     "Pressure ID for thermo does not exist");
+        pcompute = modify->get_compute_by_id(id_compute[index_press_scalar]);
+        if (!pcompute) error->all(FLERR, "Pressure compute {} for thermo output does not exist",
+                                  id_compute[index_press_scalar]);
       } else if (index_press_vector >= 0) {
-        icompute = modify->find_compute(id_compute[index_press_vector]);
-        if (icompute < 0) error->all(FLERR,
-                                     "Pressure ID for thermo does not exist");
-      } else icompute = modify->find_compute("thermo_press");
-
-      modify->compute[icompute]->reset_extra_compute_fix(arg[iarg+1]);
+        pcompute = modify->get_compute_by_id(id_compute[index_press_vector]);
+        if (!pcompute) error->all(FLERR,"Pressure compute {} for thermo output does not exist",
+                                  id_compute[index_press_vector]);
+      } else pcompute = modify->get_compute_by_id("thermo_press");
 
+      pcompute->reset_extra_compute_fix(arg[iarg+1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg],"press") == 0) {
@@ -513,13 +507,12 @@ void Thermo::modify_params(int narg, char **arg)
         id_compute[index_press_vector] = utils::strdup(arg[iarg+1]);
       }
 
-      int icompute = modify->find_compute(arg[iarg+1]);
-      if (icompute < 0) error->all(FLERR,
-                                   "Could not find thermo_modify pressure ID");
-      pressure = modify->compute[icompute];
+      pressure = modify->get_compute_by_id(arg[iarg+1]);
+      if (!pressure)
+        error->all(FLERR,"Could not find thermo_modify pressure compute {}",arg[iarg+1]);
 
       if (pressure->pressflag == 0)
-        error->all(FLERR,"Thermo_modify pressure ID does not compute pressure");
+        error->all(FLERR,"Thermo_modify compute {} does not compute pressure",arg[iarg+1]);
 
       iarg += 2;
 
@@ -912,24 +905,24 @@ void Thermo::parse_fields(char *str)
       argindex2[nfield] = (argi.get_dim() > 1) ? argi.get_index2() : 0;
 
       if (argi.get_type() == ArgInfo::COMPUTE) {
-        int n = modify->find_compute(argi.get_name());
-        if (n < 0) error->all(FLERR,"Could not find thermo custom compute ID");
-        if (argindex1[nfield] == 0 && modify->compute[n]->scalar_flag == 0)
+        auto icompute = modify->get_compute_by_id(argi.get_name());
+        if (!icompute) error->all(FLERR,"Could not find thermo custom compute ID");
+        if (argindex1[nfield] == 0 && icompute->scalar_flag == 0)
           error->all(FLERR,"Thermo compute does not compute scalar");
         if (argindex1[nfield] > 0 && argindex2[nfield] == 0) {
-          if (modify->compute[n]->vector_flag == 0)
+          if (icompute->vector_flag == 0)
             error->all(FLERR,"Thermo compute does not compute vector");
-          if (argindex1[nfield] > modify->compute[n]->size_vector &&
-              modify->compute[n]->size_vector_variable == 0)
+          if (argindex1[nfield] > icompute->size_vector &&
+              icompute->size_vector_variable == 0)
             error->all(FLERR,"Thermo compute vector is accessed out-of-range");
         }
         if (argindex1[nfield] > 0 && argindex2[nfield] > 0) {
-          if (modify->compute[n]->array_flag == 0)
+          if (icompute->array_flag == 0)
             error->all(FLERR,"Thermo compute does not compute array");
-          if (argindex1[nfield] > modify->compute[n]->size_array_rows &&
-              modify->compute[n]->size_array_rows_variable == 0)
+          if (argindex1[nfield] > icompute->size_array_rows &&
+              icompute->size_array_rows_variable == 0)
             error->all(FLERR,"Thermo compute array is accessed out-of-range");
-          if (argindex2[nfield] > modify->compute[n]->size_array_cols)
+          if (argindex2[nfield] > icompute->size_array_cols)
             error->all(FLERR,"Thermo compute array is accessed out-of-range");
         }
 
@@ -942,24 +935,24 @@ void Thermo::parse_fields(char *str)
         addfield(word.c_str(), &Thermo::compute_compute, FLOAT);
 
       } else if (argi.get_type() == ArgInfo::FIX) {
-        int n = modify->find_fix(argi.get_name());
-        if (n < 0) error->all(FLERR,"Could not find thermo custom fix ID");
-        if (argindex1[nfield] == 0 && modify->fix[n]->scalar_flag == 0)
+        auto ifix = modify->get_fix_by_id(argi.get_name());
+        if (!ifix) error->all(FLERR,"Could not find thermo custom fix ID");
+        if (argindex1[nfield] == 0 && ifix->scalar_flag == 0)
           error->all(FLERR,"Thermo fix does not compute scalar");
         if (argindex1[nfield] > 0 && argindex2[nfield] == 0) {
-          if (modify->fix[n]->vector_flag == 0)
+          if (ifix->vector_flag == 0)
             error->all(FLERR,"Thermo fix does not compute vector");
-          if (argindex1[nfield] > modify->fix[n]->size_vector &&
-              modify->fix[n]->size_vector_variable == 0)
+          if (argindex1[nfield] > ifix->size_vector &&
+              ifix->size_vector_variable == 0)
             error->all(FLERR,"Thermo fix vector is accessed out-of-range");
         }
         if (argindex1[nfield] > 0 && argindex2[nfield] > 0) {
-          if (modify->fix[n]->array_flag == 0)
+          if (ifix->array_flag == 0)
             error->all(FLERR,"Thermo fix does not compute array");
-          if (argindex1[nfield] > modify->fix[n]->size_array_rows &&
-              modify->fix[n]->size_array_rows_variable == 0)
+          if (argindex1[nfield] > ifix->size_array_rows &&
+              ifix->size_array_rows_variable == 0)
             error->all(FLERR,"Thermo fix array is accessed out-of-range");
-          if (argindex2[nfield] > modify->fix[n]->size_array_cols)
+          if (argindex2[nfield] > ifix->size_array_cols)
             error->all(FLERR,"Thermo fix array is accessed out-of-range");
         }
 
@@ -971,11 +964,9 @@ void Thermo::parse_fields(char *str)
         if (n < 0)
           error->all(FLERR,"Could not find thermo custom variable name");
         if (argindex1[nfield] == 0 && input->variable->equalstyle(n) == 0)
-          error->all(FLERR,
-                     "Thermo custom variable is not equal-style variable");
+          error->all(FLERR,"Thermo custom variable is not equal-style variable");
         if (argindex1[nfield] && input->variable->vectorstyle(n) == 0)
-          error->all(FLERR,
-                     "Thermo custom variable is not vector-style variable");
+          error->all(FLERR,"Thermo custom variable is not vector-style variable");
         if (argindex2[nfield])
           error->all(FLERR,"Thermo custom variable cannot have two indices");
 
diff --git a/src/utils.cpp b/src/utils.cpp
index 507500f116..dc96dd4c1f 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -583,12 +583,11 @@ int utils::expand_args(const char *file, int line, int narg, char **arg, int mod
       // compute
 
       if (word[0] == 'c') {
-        int icompute = lmp->modify->find_compute(id);
+        auto compute = lmp->modify->get_compute_by_id(id);
 
         // check for global vector/array, peratom array, local array
 
-        if (icompute >= 0) {
-          Compute *compute = lmp->modify->compute[icompute];
+        if (compute) {
           if (mode == 0 && compute->vector_flag) {
             nmax = compute->size_vector;
             expandflag = 1;
@@ -607,13 +606,11 @@ int utils::expand_args(const char *file, int line, int narg, char **arg, int mod
         // fix
 
       } else if (word[0] == 'f') {
-        int ifix = lmp->modify->find_fix(id);
+        auto fix = lmp->modify->get_fix_by_id(id);
 
         // check for global vector/array, peratom array, local array
 
-        if (ifix >= 0) {
-          Fix *fix = lmp->modify->fix[ifix];
-
+        if (fix) {
           if (mode == 0 && fix->vector_flag) {
             nmax = fix->size_vector;
             expandflag = 1;