diff --git a/src/MISC/fix_srp.cpp b/src/MISC/fix_srp.cpp
index f4f9abc629..e82e98448f 100644
--- a/src/MISC/fix_srp.cpp
+++ b/src/MISC/fix_srp.cpp
@@ -116,7 +116,7 @@ void FixSRP::init()
   // because this fix's pre_exchange() creates per-atom data structure
   // that data must be current for atom migration to carry it along
 
-  for (auto ifix : modify->get_fix_list()) {
+  for (auto &ifix : modify->get_fix_list()) {
     if (ifix == this) break;
     if (ifix->pre_exchange_migrate)
       error->all(FLERR,"Fix {} comes after a fix which migrates atoms in pre_exchange", style);
diff --git a/src/MISC/pair_srp_react.cpp b/src/MISC/pair_srp_react.cpp
index 25975c7fb0..4a3f260fe0 100644
--- a/src/MISC/pair_srp_react.cpp
+++ b/src/MISC/pair_srp_react.cpp
@@ -72,7 +72,7 @@ PairSRPREACT::PairSRPREACT(LAMMPS *lmp) :
 
   // pair srp/react has its own fix, hence delete fix srp instance
   // created in the constructor of pair srp
-  for (auto ifix : modify->get_fix_by_style("SRP"))
+  for (auto &ifix : modify->get_fix_by_style("SRP"))
     modify->delete_fix(ifix->id);
 
   // similar to fix SRP, create fix SRP REACT instance here with unique fix id
diff --git a/src/PLUGIN/plugin.cpp b/src/PLUGIN/plugin.cpp
index 5451cd31e2..97c3b36cce 100644
--- a/src/PLUGIN/plugin.cpp
+++ b/src/PLUGIN/plugin.cpp
@@ -369,7 +369,7 @@ void plugin_unload(const char *style, const char *name, LAMMPS *lmp)
 
     // must delete all compute instances using this compute style
 
-    for (auto icompute : lmp->modify->get_compute_by_style(name))
+    for (auto &icompute : lmp->modify->get_compute_by_style(name))
       lmp->modify->delete_compute(icompute->id);
 
   } else if (pstyle == "fix") {
@@ -380,7 +380,7 @@ void plugin_unload(const char *style, const char *name, LAMMPS *lmp)
 
     // must delete all fix instances using this fix style
 
-    for (auto ifix : lmp->modify->get_fix_by_style(name)) lmp->modify->delete_fix(ifix->id);
+    for (auto &ifix : lmp->modify->get_fix_by_style(name)) lmp->modify->delete_fix(ifix->id);
 
   } else if (pstyle == "region") {
 
@@ -388,7 +388,7 @@ void plugin_unload(const char *style, const char *name, LAMMPS *lmp)
     auto found = region_map->find(name);
     if (found != region_map->end()) region_map->erase(name);
 
-    for (auto iregion : lmp->domain->get_region_by_style(name)) lmp->domain->delete_region(iregion);
+    for (auto &iregion : lmp->domain->get_region_by_style(name)) lmp->domain->delete_region(iregion);
 
   } else if (pstyle == "command") {
 
diff --git a/src/POEMS/fix_poems.cpp b/src/POEMS/fix_poems.cpp
index b2b0366b9e..312208f4d9 100644
--- a/src/POEMS/fix_poems.cpp
+++ b/src/POEMS/fix_poems.cpp
@@ -354,7 +354,7 @@ void FixPOEMS::init()
 
   if (earlyflag) {
     bool pflag = false;
-    for (auto ifix : modify->get_fix_list()) {
+    for (auto &ifix : modify->get_fix_list()) {
       if (utils::strmatch(ifix->style, "^poems")) pflag = true;
       if (pflag && (ifix->setmask() & POST_FORCE) && !ifix->rigid_flag)
         if (comm->me == 0)
@@ -365,7 +365,7 @@ void FixPOEMS::init()
 
   // error if npt,nph fix comes before rigid fix
   bool pflag = false;
-  for (auto ifix : modify->get_fix_list()) {
+  for (auto &ifix : modify->get_fix_list()) {
     if (!pflag && utils::strmatch(ifix->style, "np[th]"))
       error->all(FLERR, "POEMS fix must come before NPT/NPH fix");
     if (utils::strmatch(ifix->style, "^poems")) pflag = true;
diff --git a/src/REPLICA/hyper.cpp b/src/REPLICA/hyper.cpp
index f13711f213..c2e968e41b 100644
--- a/src/REPLICA/hyper.cpp
+++ b/src/REPLICA/hyper.cpp
@@ -152,10 +152,10 @@ void Hyper::command(int narg, char **arg)
 
   // cannot use hyper with time-dependent fixes or regions
 
-  for (auto ifix : modify->get_fix_list())
+  for (auto &ifix : modify->get_fix_list())
     if (ifix->time_depend) error->all(FLERR,"Cannot use hyper with a time-dependent fix defined");
 
-  for (auto reg : domain->get_region_list())
+  for (auto &reg : domain->get_region_list())
     if (reg->dynamic_check())
       error->all(FLERR,"Cannot use hyper with a time-dependent region defined");
 
@@ -177,7 +177,7 @@ void Hyper::command(int narg, char **arg)
 
   fix_event->store_state_quench();
   quench(1);
-  if (dumpflag) for (auto idump : dumplist) idump->write();
+  if (dumpflag) for (auto &idump : dumplist) idump->write();
   fix_event->store_event();
   if (hyperenable) fix_hyper->build_bond_list(0);
   fix_event->restore_state_quench();
@@ -206,7 +206,7 @@ void Hyper::command(int narg, char **arg)
       nevent++;
       nevent_atoms += ecount;
 
-      if (dumpflag) for (auto idump : dumplist) idump->write();
+      if (dumpflag) for (auto &idump : dumplist) idump->write();
       fix_event->store_event();
       if (hyperenable) fix_hyper->build_bond_list(ecount);
 
diff --git a/src/REPLICA/prd.cpp b/src/REPLICA/prd.cpp
index 79d09080ec..a658dba853 100644
--- a/src/REPLICA/prd.cpp
+++ b/src/REPLICA/prd.cpp
@@ -229,10 +229,10 @@ void PRD::command(int narg, char **arg)
 
   // cannot use PRD with time-dependent fixes or regions
 
-  for (auto ifix : modify->get_fix_list())
+  for (auto &ifix : modify->get_fix_list())
     if (ifix->time_depend) error->all(FLERR,"Cannot use PRD with a time-dependent fix defined");
 
-  for (auto reg : domain->get_region_list())
+  for (auto &reg : domain->get_region_list())
     if (reg->dynamic_check())
       error->all(FLERR,"Cannot use PRD with a time-dependent region defined");
 
diff --git a/src/RIGID/fix_rigid.cpp b/src/RIGID/fix_rigid.cpp
index f21d46d576..eecd59cfa0 100644
--- a/src/RIGID/fix_rigid.cpp
+++ b/src/RIGID/fix_rigid.cpp
@@ -684,13 +684,13 @@ void FixRigid::init()
   // if earlyflag, warn if any post-force fixes come after a rigid fix
 
   int count = 0;
-  for (auto ifix : modify->get_fix_list())
+  for (auto &ifix : modify->get_fix_list())
     if (ifix->rigid_flag) count++;
   if (count > 1 && me == 0) error->warning(FLERR,"More than one fix rigid");
 
   if (earlyflag) {
     bool rflag = false;
-    for (auto ifix : modify->get_fix_list()) {
+    for (auto &ifix : modify->get_fix_list()) {
       if (ifix->rigid_flag) rflag = true;
       if ((comm->me == 0) && rflag && (ifix->setmask() & POST_FORCE) && !ifix->rigid_flag)
         error->warning(FLERR,"Fix {} with ID {} alters forces after fix rigid",
@@ -713,7 +713,7 @@ void FixRigid::init()
   //  error if a fix changing the box comes before rigid fix
 
   bool boxflag = false;
-  for (auto ifix : modify->get_fix_list()) {
+  for (auto &ifix : modify->get_fix_list()) {
     if (boxflag && utils::strmatch(ifix->style,"^rigid"))
         error->all(FLERR,"Rigid fixes must come before any box changing fix");
     if (ifix->box_change) boxflag = true;
diff --git a/src/RIGID/fix_rigid_small.cpp b/src/RIGID/fix_rigid_small.cpp
index ec3e58d5f5..2eb3c1a63c 100644
--- a/src/RIGID/fix_rigid_small.cpp
+++ b/src/RIGID/fix_rigid_small.cpp
@@ -529,13 +529,13 @@ void FixRigidSmall::init()
   // if earlyflag, warn if any post-force fixes come after a rigid fix
 
   int count = 0;
-  for (auto ifix : modify->get_fix_list())
+  for (auto &ifix : modify->get_fix_list())
     if (ifix->rigid_flag) count++;
   if (count > 1 && me == 0) error->warning(FLERR,"More than one fix rigid");
 
   if (earlyflag) {
     bool rflag = false;
-    for (auto ifix : modify->get_fix_list()) {
+    for (auto &ifix : modify->get_fix_list()) {
       if (ifix->rigid_flag) rflag = true;
       if ((comm->me == 0) && rflag && (ifix->setmask() & POST_FORCE) && !ifix->rigid_flag)
         error->warning(FLERR,"Fix {} with ID {} alters forces after fix rigid/small",
@@ -558,7 +558,7 @@ void FixRigidSmall::init()
   // error if a fix changing the box comes before rigid fix
 
   bool boxflag = false;
-  for (auto ifix : modify->get_fix_list()) {
+  for (auto &ifix : modify->get_fix_list()) {
     if (boxflag && utils::strmatch(ifix->style,"^rigid"))
         error->all(FLERR,"Rigid fixes must come before any box changing fix");
     if (ifix->box_change) boxflag = true;
diff --git a/src/RIGID/fix_shake.cpp b/src/RIGID/fix_shake.cpp
index 6673472458..1ff61c8856 100644
--- a/src/RIGID/fix_shake.cpp
+++ b/src/RIGID/fix_shake.cpp
@@ -360,7 +360,7 @@ void FixShake::init()
 
   // error if a fix changing the box comes before shake fix
   bool boxflag = false;
-  for (auto ifix : modify->get_fix_list()) {
+  for (auto &ifix : modify->get_fix_list()) {
    if (boxflag && utils::strmatch(ifix->style,pattern))
      error->all(FLERR,"Fix {} must come before any box changing fix", style);
     if (ifix->box_change) boxflag = true;
diff --git a/src/domain.cpp b/src/domain.cpp
index 48631a0899..c3dacc0201 100644
--- a/src/domain.cpp
+++ b/src/domain.cpp
@@ -125,7 +125,7 @@ Domain::~Domain()
 {
   if (copymode) return;
 
-  for (auto reg : regions) delete reg;
+  for (auto &reg : regions) delete reg;
   regions.clear();
   delete lattice;
   delete region_map;
@@ -190,7 +190,7 @@ void Domain::init()
 
   // region inits
 
-  for (auto reg : regions) reg->init();
+  for (auto &reg : regions) reg->init();
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/fix_deform.cpp b/src/fix_deform.cpp
index de24d9d652..bc6e61a69e 100644
--- a/src/fix_deform.cpp
+++ b/src/fix_deform.cpp
@@ -882,7 +882,7 @@ void FixDeform::end_of_step()
       if (mask[i] & groupbit)
         domain->x2lamda(x[i],x[i]);
 
-    for (auto ifix : rfix)
+    for (auto &ifix : rfix)
       ifix->deform(0);
   }
 
@@ -921,7 +921,7 @@ void FixDeform::end_of_step()
       if (mask[i] & groupbit)
         domain->lamda2x(x[i],x[i]);
 
-    for (auto ifix : rfix)
+    for (auto &ifix : rfix)
       ifix->deform(1);
   }
 
diff --git a/src/utils.cpp b/src/utils.cpp
index 9e6aa8e5d1..6483997dc9 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -1129,7 +1129,7 @@ bool utils::is_integer(const std::string &str)
 {
   if (str.empty()) return false;
 
-  for (auto c : str) {
+  for (const auto &c : str) {
     if (isdigit(c) || c == '-' || c == '+') continue;
     return false;
   }
@@ -1144,7 +1144,7 @@ bool utils::is_double(const std::string &str)
 {
   if (str.empty()) return false;
 
-  for (auto c : str) {
+  for (const auto &c : str) {
     if (isdigit(c)) continue;
     if (c == '-' || c == '+' || c == '.') continue;
     if (c == 'e' || c == 'E') continue;
@@ -1161,7 +1161,7 @@ bool utils::is_id(const std::string &str)
 {
   if (str.empty()) return false;
 
-  for (auto c : str) {
+  for (const auto &c : str) {
     if (isalnum(c) || (c == '_')) continue;
     return false;
   }
@@ -1178,7 +1178,7 @@ int utils::is_type(const std::string &str)
 
   bool numeric = true;
   int nstar = 0;
-  for (auto c : str) {
+  for (const auto &c : str) {
     if (isdigit(c)) continue;
     if (c == '*') {
       ++nstar;