diff --git a/src/OPENMP/respa_omp.cpp b/src/OPENMP/respa_omp.cpp
index cc4d3925b1..c6500185ad 100644
--- a/src/OPENMP/respa_omp.cpp
+++ b/src/OPENMP/respa_omp.cpp
@@ -67,21 +67,23 @@ void RespaOMP::init()
 void RespaOMP::setup(int flag)
 {
   if (comm->me == 0 && screen) {
-    fprintf(screen,"Setting up r-RESPA/omp run ...\n");
+    std::string mesg = "Setting up r-RESPA/omp run ...\n";
     if (flag) {
-      fprintf(screen,"  Unit style    : %s\n", update->unit_style);
-      fprintf(screen,"  Current step  : " BIGINT_FORMAT "\n", update->ntimestep);
-      fprintf(screen,"  Time steps    :");
-      for (int ilevel=0; ilevel < nlevels; ++ilevel)
-        fprintf(screen," %d:%g",ilevel+1, step[ilevel]);
-      fprintf(screen,"\n  r-RESPA fixes :");
-      for (int l=0; l < modify->n_post_force_respa; ++l) {
-        Fix *f = modify->fix[modify->list_post_force_respa[l]];
+      mesg += fmt::format("  Unit style    : {}\n", update->unit_style);
+      mesg += fmt::format("  Current step  : {}\n", update->ntimestep);
+
+      mesg += "  Time steps    :";
+      for (int ilevel = 0; ilevel < nlevels; ++ilevel)
+        mesg += fmt::format(" {}:{}", ilevel + 1, step[ilevel]);
+
+      mesg += "\n  r-RESPA fixes :";
+      for (int l = 0; l < modify->n_post_force_respa; ++l) {
+        Fix *f = modify->get_fix_by_index(modify->list_post_force_respa[l]);
         if (f->respa_level >= 0)
-          fprintf(screen," %d:%s[%s]",
-                  MIN(f->respa_level+1,nlevels),f->style,f->id);
+          mesg += fmt::format(" {}:{}[{}]", MIN(f->respa_level + 1, nlevels), f->style, f->id);
       }
-      fprintf(screen,"\n");
+      mesg += "\n";
+      fputs(mesg.c_str(), screen);
       timer->print_timeout(screen);
     }
   }
diff --git a/src/fix_ave_time.cpp b/src/fix_ave_time.cpp
index a3da6e6a3d..836121fdc2 100644
--- a/src/fix_ave_time.cpp
+++ b/src/fix_ave_time.cpp
@@ -243,10 +243,8 @@ FixAveTime::FixAveTime(LAMMPS *lmp, int narg, char **arg) :
   if (any_variable_length &&
       (nrepeat > 1 || ave == RUNNING || ave == WINDOW)) {
     for (int i = 0; i < nvalues; i++)
-      if (varlen[i] && which[i] == ArgInfo::COMPUTE) {
-        int icompute = modify->find_compute(ids[i]);
-        modify->compute[icompute]->lock_enable();
-      }
+      if (varlen[i] && which[i] == ArgInfo::COMPUTE)
+        modify->get_compute_by_id(ids[i])->lock_enable();
     lockforever = 0;
   }
 
diff --git a/src/fix_vector.cpp b/src/fix_vector.cpp
index 9066182ed1..e916b8e9fe 100644
--- a/src/fix_vector.cpp
+++ b/src/fix_vector.cpp
@@ -65,64 +65,54 @@ FixVector::FixVector(LAMMPS *lmp, int narg, char **arg) :
 
   // setup and error check
   // for fix inputs, check that fix frequency is acceptable
-
-  for (int i = 0; i < nvalues; i++) {
-    if (which[i] == ArgInfo::COMPUTE) {
-      int icompute = modify->find_compute(ids[i]);
-      if (icompute < 0)
-        error->all(FLERR,"Compute ID for fix vector does not exist");
-      if (argindex[i] == 0 && modify->compute[icompute]->scalar_flag == 0)
-        error->all(FLERR,"Fix vector compute does not calculate a scalar");
-      if (argindex[i] && modify->compute[icompute]->vector_flag == 0)
-        error->all(FLERR,"Fix vector compute does not calculate a vector");
-      if (argindex[i] && argindex[i] > modify->compute[icompute]->size_vector)
-        error->all(FLERR,
-                   "Fix vector compute vector is accessed out-of-range");
-
-    } else if (which[i] == ArgInfo::FIX) {
-      int ifix = modify->find_fix(ids[i]);
-      if (ifix < 0)
-        error->all(FLERR,"Fix ID for fix vector does not exist");
-      if (argindex[i] == 0 && modify->fix[ifix]->scalar_flag == 0)
-        error->all(FLERR,"Fix vector fix does not calculate a scalar");
-      if (argindex[i] && modify->fix[ifix]->vector_flag == 0)
-        error->all(FLERR,"Fix vector fix does not calculate a vector");
-      if (argindex[i] && argindex[i] > modify->fix[ifix]->size_vector)
-        error->all(FLERR,"Fix vector fix vector is accessed out-of-range");
-      if (nevery % modify->fix[ifix]->global_freq)
-        error->all(FLERR,
-                   "Fix for fix vector not computed at compatible time");
-
-    } else if (which[i] == ArgInfo::VARIABLE) {
-      int ivariable = input->variable->find(ids[i]);
-      if (ivariable < 0)
-        error->all(FLERR,"Variable name for fix vector does not exist");
-      if (argindex[i] == 0 && input->variable->equalstyle(ivariable) == 0)
-        error->all(FLERR,"Fix vector variable is not equal-style variable");
-      if (argindex[i] && input->variable->vectorstyle(ivariable) == 0)
-        error->all(FLERR,"Fix vector variable is not vector-style variable");
-    }
-  }
-
   // this fix produces either a global vector or array
   // intensive/extensive flags set by compute,fix,variable that produces value
 
   int value,finalvalue;
   for (int i = 0; i < nvalues; i++) {
     if (which[i] == ArgInfo::COMPUTE) {
-      Compute *compute = modify->compute[modify->find_compute(ids[i])];
-      if (argindex[0] == 0) value = compute->extscalar;
-      else if (compute->extvector >= 0) value = compute->extvector;
-      else value = compute->extlist[argindex[0]-1];
+      auto icompute = modify->get_compute_by_id(ids[i]);
+      if (!icompute) error->all(FLERR,"Compute ID {} for fix vector does not exist",ids[i]);
+      if (argindex[i] == 0 && icompute->scalar_flag == 0)
+        error->all(FLERR,"Fix vector compute {} does not calculate a scalar",ids[i]);
+      if (argindex[i] && icompute->vector_flag == 0)
+        error->all(FLERR,"Fix vector compute {} does not calculate a vector",ids[i]);
+      if (argindex[i] && argindex[i] > icompute->size_vector)
+        error->all(FLERR,"Fix vector compute {} vector is accessed out-of-range",ids[i]);
+
+      if (argindex[i] == 0) value = icompute->extscalar;
+      else if (icompute->extvector >= 0) value = icompute->extvector;
+      else value = icompute->extlist[argindex[i]-1];
+
     } else if (which[i] == ArgInfo::FIX) {
-      Fix *fix = modify->fix[modify->find_fix(ids[i])];
-      if (argindex[i] == 0) value = fix->extvector;
-      else value = fix->extarray;
-    } else if (which[i] == ArgInfo::VARIABLE) value = 0;
+      auto ifix = modify->get_fix_by_id(ids[i]);
+      if (!ifix) error->all(FLERR,"Fix ID {} for fix vector does not exist",ids[i]);
+      if (argindex[i] == 0 && ifix->scalar_flag == 0)
+        error->all(FLERR,"Fix vector fix {} does not calculate a scalar",ids[i]);
+      if (argindex[i] && ifix->vector_flag == 0)
+        error->all(FLERR,"Fix vector fix {} does not calculate a vector",ids[i]);
+      if (argindex[i] && argindex[i] > ifix->size_vector)
+        error->all(FLERR,"Fix vector fix {} vector is accessed out-of-range",ids[i]);
+      if (nevery % ifix->global_freq)
+        error->all(FLERR,"Fix for fix {} vector not computed at compatible time",ids[i]);
+
+      if (argindex[i] == 0) value = ifix->extvector;
+      else value = ifix->extarray;
+
+    } else if (which[i] == ArgInfo::VARIABLE) {
+      int ivariable = input->variable->find(ids[i]);
+      if (ivariable < 0)
+        error->all(FLERR,"Variable name {} for fix vector does not exist",ids[i]);
+      if (argindex[i] == 0 && input->variable->equalstyle(ivariable) == 0)
+        error->all(FLERR,"Fix vector variable {} is not equal-style variable",ids[i]);
+      if (argindex[i] && input->variable->vectorstyle(ivariable) == 0)
+        error->all(FLERR,"Fix vector variable {} is not vector-style variable",ids[i]);
+      value = 0;
+    }
+
     if (i == 0) finalvalue = value;
     else if (value != finalvalue)
-      error->all(FLERR,"Fix vector cannot set output array "
-                 "intensive/extensive from these inputs");
+      error->all(FLERR,"Fix vector cannot set output array intensive/extensive from these inputs");
   }
 
   if (nvalues == 1) {
@@ -191,14 +181,12 @@ void FixVector::init()
   for (int i = 0; i < nvalues; i++) {
     if (which[i] == ArgInfo::COMPUTE) {
       int icompute = modify->find_compute(ids[i]);
-      if (icompute < 0)
-        error->all(FLERR,"Compute ID for fix vector does not exist");
+      if (icompute < 0) error->all(FLERR,"Compute ID {} for fix vector does not exist",id[i]);
       value2index[i] = icompute;
 
     } else if (which[i] == ArgInfo::FIX) {
       int ifix = modify->find_fix(ids[i]);
-      if (ifix < 0)
-        error->all(FLERR,"Fix ID for fix vector does not exist");
+      if (ifix < 0) error->all(FLERR,"Fix ID {} for fix vector does not exist",id[i]);
       value2index[i] = ifix;
 
     } else if (which[i] == ArgInfo::VARIABLE) {
@@ -254,7 +242,7 @@ void FixVector::end_of_step()
     // invoke compute if not previously invoked
 
     if (which[i] == ArgInfo::COMPUTE) {
-      Compute *compute = modify->compute[m];
+      auto compute = modify->get_compute_by_index(m);
 
       if (argindex[i] == 0) {
         if (!(compute->invoked_flag & Compute::INVOKED_SCALAR)) {
@@ -274,9 +262,9 @@ void FixVector::end_of_step()
 
     } else if (which[i] == ArgInfo::FIX) {
       if (argindex[i] == 0)
-        result[i] = modify->fix[m]->compute_scalar();
+        result[i] = modify->get_fix_by_index(m)->compute_scalar();
       else
-        result[i] = modify->fix[m]->compute_vector(argindex[i]-1);
+        result[i] = modify->get_fix_by_index(m)->compute_vector(argindex[i]-1);
 
     // evaluate equal-style or vector-style variable
 
diff --git a/src/modify.h b/src/modify.h
index 60365cd56c..c4f8c2ef02 100644
--- a/src/modify.h
+++ b/src/modify.h
@@ -113,6 +113,7 @@ class Modify : protected Pointers {
   int find_fix(const std::string &);
   // new API
   Fix *get_fix_by_id(const std::string &) const;
+  Fix *get_fix_by_index(int idx) const { return fix[idx]; }
   const std::vector<Fix *> get_fix_by_style(const std::string &) const;
   const std::vector<Fix *> &get_fix_list();
 
@@ -126,6 +127,7 @@ class Modify : protected Pointers {
   int find_compute(const std::string &);
   // new API
   Compute *get_compute_by_id(const std::string &) const;
+  Compute *get_compute_by_index(int idx) const { return compute[idx]; }
   const std::vector<Compute *> get_compute_by_style(const std::string &) const;
   const std::vector<Compute *> &get_compute_list();
 
diff --git a/src/respa.cpp b/src/respa.cpp
index af8b0c21ac..9690105d40 100644
--- a/src/respa.cpp
+++ b/src/respa.cpp
@@ -373,7 +373,7 @@ void Respa::setup(int flag)
 
       mesg += "\n  r-RESPA fixes :";
       for (int l = 0; l < modify->n_post_force_respa; ++l) {
-        Fix *f = modify->fix[modify->list_post_force_respa[l]];
+        Fix *f = modify->get_fix_by_index(modify->list_post_force_respa[l]);
         if (f->respa_level >= 0)
           mesg += fmt::format(" {}:{}[{}]", MIN(f->respa_level + 1, nlevels), f->style, f->id);
       }
diff --git a/src/special.cpp b/src/special.cpp
index 9f480da78f..ceba160be2 100644
--- a/src/special.cpp
+++ b/src/special.cpp
@@ -1290,9 +1290,8 @@ int Special::rendezvous_pairs(int n, char *inbuf, int &flag, int *&proclist,
 
 void Special::fix_alteration()
 {
-  for (int ifix = 0; ifix < modify->nfix; ifix++)
-    if (modify->fix[ifix]->special_alter_flag)
-      modify->fix[ifix]->rebuild_special();
+  for (const auto &ifix : modify->get_fix_list())
+    if (ifix->special_alter_flag) ifix->rebuild_special();
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/update.cpp b/src/update.cpp
index 72b60c3d3b..95dc47573e 100644
--- a/src/update.cpp
+++ b/src/update.cpp
@@ -488,41 +488,29 @@ void Update::reset_timestep(bigint newstep)
 
   output->reset_timestep(ntimestep);
 
-  for (int i = 0; i < modify->nfix; i++) {
-    if (modify->fix[i]->time_depend)
-      error->all(FLERR,
-                 "Cannot reset timestep with a time-dependent fix defined");
-  }
+  for (const auto &ifix : modify->get_fix_list())
+    if (ifix->time_depend)
+      error->all(FLERR, "Cannot reset timestep with time-dependent fix {} defined",ifix->style);
 
   // reset eflag/vflag global so no commands will think eng/virial are current
 
   eflag_global = vflag_global = -1;
 
-  // reset invoked flags of computes,
-  // so no commands will think they are current between runs
-
-  for (int i = 0; i < modify->ncompute; i++) {
-    modify->compute[i]->invoked_scalar = -1;
-    modify->compute[i]->invoked_vector = -1;
-    modify->compute[i]->invoked_array = -1;
-    modify->compute[i]->invoked_peratom = -1;
-    modify->compute[i]->invoked_local = -1;
-  }
-
+  // reset invoked flags of computes, so no commands will think they are current between runs
   // clear timestep list of computes that store future invocation times
 
-  for (int i = 0; i < modify->ncompute; i++)
-    if (modify->compute[i]->timeflag) modify->compute[i]->clearstep();
+  for (const auto &icompute : modify->get_compute_list()) {
+    icompute->invoked_scalar = -1;
+    icompute->invoked_vector = -1;
+    icompute->invoked_array = -1;
+    icompute->invoked_peratom = -1;
+    icompute->invoked_local = -1;
+    if (icompute->timeflag) icompute->clearstep();
+  }
 
   // Neighbor Bin/Stencil/Pair classes store timestamps that need to be cleared
 
   neighbor->reset_timestep(ntimestep);
-
-  // NOTE: 7Jun12, adding rerun command, don't think this is required
-
-  //for (int i = 0; i < domain->nregion; i++)
-  //  if (domain->regions[i]->dynamic_check())
-  //    error->all(FLERR,"Cannot reset timestep with a dynamic region defined");
 }
 
 /* ----------------------------------------------------------------------