diff --git a/src/atom.cpp b/src/atom.cpp
index 864c7af638..90dce44764 100644
--- a/src/atom.cpp
+++ b/src/atom.cpp
@@ -12,6 +12,7 @@
 ------------------------------------------------------------------------- */
 
 #include "atom.h"
+#include "atom_vec.h"
 #include "style_atom.h"  // IWYU pragma: keep
 
 #include "comm.h"
diff --git a/src/body.h b/src/body.h
index 1c742d3e65..c9c289792e 100644
--- a/src/body.h
+++ b/src/body.h
@@ -15,7 +15,7 @@
 #define LMP_BODY_H
 
 #include "pointers.h"
-#include "atom_vec_body.h"
+#include "atom_vec_body.h"   // IWYU pragma: keep
 
 namespace LAMMPS_NS {
 
diff --git a/src/compute_orientorder_atom.cpp b/src/compute_orientorder_atom.cpp
index 84ae20e92a..7323b6ef01 100644
--- a/src/compute_orientorder_atom.cpp
+++ b/src/compute_orientorder_atom.cpp
@@ -36,7 +36,6 @@
 
 using namespace LAMMPS_NS;
 using namespace MathConst;
-using namespace std;
 
 #ifdef DBL_EPSILON
   #define MY_EPSILON (10.0*DBL_EPSILON)
diff --git a/src/compute_pressure.cpp b/src/compute_pressure.cpp
index b57b53f522..90e4f9b39c 100644
--- a/src/compute_pressure.cpp
+++ b/src/compute_pressure.cpp
@@ -13,22 +13,23 @@
 
 #include "compute_pressure.h"
 
-#include <cstring>
+#include "angle.h"
 #include "atom.h"
-#include "update.h"
+#include "bond.h"
+#include "dihedral.h"
 #include "domain.h"
-#include "modify.h"
+#include "error.h"
 #include "fix.h"
 #include "force.h"
-#include "pair.h"
-#include "pair_hybrid.h"
-#include "bond.h"
-#include "angle.h"
-#include "dihedral.h"
 #include "improper.h"
 #include "kspace.h"
-#include "error.h"
+#include "modify.h"
+#include "pair.h"
+#include "pair_hybrid.h"
+#include "update.h"
 
+#include <cctype>
+#include <cstring>
 using namespace LAMMPS_NS;
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/domain.h b/src/domain.h
index f3c8026e72..d807463bf3 100644
--- a/src/domain.h
+++ b/src/domain.h
@@ -20,6 +20,7 @@
 #include <map>
 
 namespace LAMMPS_NS {
+  class Region;
 
 class Domain : protected Pointers {
  public:
@@ -90,7 +91,7 @@ class Domain : protected Pointers {
 
   int nregion;                             // # of defined Regions
   int maxregion;                           // max # list can hold
-  class Region **regions;                  // list of defined Regions
+  Region **regions;                        // list of defined Regions
 
   int copymode;
   enum{NO_REMAP,X_REMAP,V_REMAP};
diff --git a/src/lattice.cpp b/src/lattice.cpp
index f41c32d239..68f3e7abe1 100644
--- a/src/lattice.cpp
+++ b/src/lattice.cpp
@@ -12,17 +12,15 @@
 ------------------------------------------------------------------------- */
 
 #include "lattice.h"
+
+#include "comm.h"
+#include "domain.h"
+#include "error.h"
+#include "memory.h"
+#include "update.h"
+
 #include <cmath>
 #include <cstring>
-#include <cstdlib>
-#include "update.h"
-#include "domain.h"
-#include "comm.h"
-#include "force.h"
-#include "memory.h"
-#include "error.h"
-#include "utils.h"
-#include "fmt/format.h"
 
 using namespace LAMMPS_NS;
 
diff --git a/src/library.cpp b/src/library.cpp
index 30d52aefe5..3f37aecd7a 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -16,33 +16,31 @@
 
 #include "library.h"
 #include <mpi.h>
-#include <cctype>
-#include <cstring>
-#include <cstdlib>
-#include <string>
-#include "universe.h"
-#include "atom_vec.h"
+
 #include "atom.h"
+#include "atom_vec.h"
+#include "comm.h"
+#include "compute.h"
 #include "domain.h"
-#include "update.h"
+#include "error.h"
+#include "fix.h"
+#include "fix_external.h"
+#include "force.h"
 #include "group.h"
+#include "info.h"
 #include "input.h"
-#include "variable.h"
+#include "memory.h"
 #include "modify.h"
+#include "neigh_list.h"
+#include "neighbor.h"
 #include "output.h"
 #include "thermo.h"
-#include "compute.h"
-#include "fix.h"
-#include "comm.h"
-#include "memory.h"
-#include "error.h"
-#include "force.h"
-#include "info.h"
-#include "fix_external.h"
-#include "neighbor.h"
-#include "neigh_list.h"
-#include "neigh_request.h"
-#include "fmt/format.h"
+#include "universe.h"
+#include "update.h"
+#include "variable.h"
+
+#include <cstring>
+#include <vector>
 
 #if defined(LAMMPS_EXCEPTIONS)
 #include "exceptions.h"
diff --git a/src/memory.cpp b/src/memory.cpp
index 6211687a7a..bbbc233bbf 100644
--- a/src/memory.cpp
+++ b/src/memory.cpp
@@ -12,9 +12,8 @@
 ------------------------------------------------------------------------- */
 
 #include "memory.h"
-#include <cstdlib>
+
 #include "error.h"
-#include "fmt/format.h"
 
 #if defined(LMP_USER_INTEL) && defined(__INTEL_COMPILER)
 #ifndef LMP_INTEL_NO_TBB
diff --git a/src/min_cg.cpp b/src/min_cg.cpp
index 46da4bfa5b..511b16cef8 100644
--- a/src/min_cg.cpp
+++ b/src/min_cg.cpp
@@ -12,12 +12,13 @@
 ------------------------------------------------------------------------- */
 
 #include "min_cg.h"
-#include <mpi.h>
-#include <cmath>
+
 #include "error.h"
-#include "update.h"
 #include "output.h"
 #include "timer.h"
+#include "update.h"
+
+#include <cmath>
 
 using namespace LAMMPS_NS;
 
diff --git a/src/min_fire_old.cpp b/src/min_fire_old.cpp
index bd76b2d1fc..6efd00a4b8 100644
--- a/src/min_fire_old.cpp
+++ b/src/min_fire_old.cpp
@@ -12,15 +12,16 @@
 ------------------------------------------------------------------------- */
 
 #include "min_fire_old.h"
-#include <mpi.h>
-#include <cmath>
-#include "universe.h"
+
 #include "atom.h"
 #include "error.h"
 #include "force.h"
-#include "update.h"
 #include "output.h"
 #include "timer.h"
+#include "universe.h"
+#include "update.h"
+
+#include <cmath>
 
 using namespace LAMMPS_NS;
 
diff --git a/src/min_hftn.cpp b/src/min_hftn.cpp
index 63432aab63..909f0b0689 100644
--- a/src/min_hftn.cpp
+++ b/src/min_hftn.cpp
@@ -18,17 +18,18 @@
 ------------------------------------------------------------------------- */
 
 #include "min_hftn.h"
-#include <mpi.h>
-#include <cmath>
-#include <cstring>
+
 #include "atom.h"
 #include "error.h"
 #include "fix_minimize.h"
 #include "modify.h"
 #include "output.h"
 #include "pair.h"
-#include "update.h"
 #include "timer.h"
+#include "update.h"
+
+#include <cmath>
+#include <cstring>
 
 using namespace LAMMPS_NS;
 
diff --git a/src/min_linesearch.cpp b/src/min_linesearch.cpp
index a7a10dcd51..7226f61614 100644
--- a/src/min_linesearch.cpp
+++ b/src/min_linesearch.cpp
@@ -22,15 +22,16 @@
 ------------------------------------------------------------------------- */
 
 #include "min_linesearch.h"
-#include <mpi.h>
-#include <cmath>
+
 #include "atom.h"
-#include "modify.h"
 #include "fix_minimize.h"
-#include "pair.h"
+#include "modify.h"
 #include "output.h"
+#include "pair.h"
 #include "thermo.h"
 
+#include <cmath>
+
 using namespace LAMMPS_NS;
 
 // ALPHA_MAX = max alpha allowed to avoid long backtracks
diff --git a/src/min_quickmin.cpp b/src/min_quickmin.cpp
index 5e7643bf3b..025e80b95d 100644
--- a/src/min_quickmin.cpp
+++ b/src/min_quickmin.cpp
@@ -12,15 +12,16 @@
 ------------------------------------------------------------------------- */
 
 #include "min_quickmin.h"
-#include <mpi.h>
-#include <cmath>
-#include "universe.h"
+
 #include "atom.h"
 #include "error.h"
 #include "force.h"
-#include "update.h"
 #include "output.h"
 #include "timer.h"
+#include "universe.h"
+#include "update.h"
+
+#include <cmath>
 
 using namespace LAMMPS_NS;
 
diff --git a/src/min_sd.cpp b/src/min_sd.cpp
index d973ec1c82..5868acbe48 100644
--- a/src/min_sd.cpp
+++ b/src/min_sd.cpp
@@ -12,11 +12,13 @@
 ------------------------------------------------------------------------- */
 
 #include "min_sd.h"
-#include <cmath>
+
 #include "error.h"
-#include "update.h"
 #include "output.h"
 #include "timer.h"
+#include "update.h"
+
+#include <cmath>
 
 using namespace LAMMPS_NS;
 
diff --git a/src/minimize.cpp b/src/minimize.cpp
index 0146390763..451412ecc8 100644
--- a/src/minimize.cpp
+++ b/src/minimize.cpp
@@ -12,13 +12,13 @@
 ------------------------------------------------------------------------- */
 
 #include "minimize.h"
+
 #include "domain.h"
-#include "update.h"
-#include "min.h"
-#include "finish.h"
-#include "timer.h"
 #include "error.h"
-#include "force.h"
+#include "finish.h"
+#include "min.h"
+#include "timer.h"
+#include "update.h"
 
 using namespace LAMMPS_NS;
 
diff --git a/src/modify.cpp b/src/modify.cpp
index 96335946ae..c88d52b623 100644
--- a/src/modify.cpp
+++ b/src/modify.cpp
@@ -12,24 +12,24 @@
 ------------------------------------------------------------------------- */
 
 #include "modify.h"
-#include <cstring>
-#include <string>
 #include "style_compute.h"
 #include "style_fix.h"
+
 #include "atom.h"
 #include "comm.h"
-#include "fix.h"
 #include "compute.h"
-#include "group.h"
-#include "update.h"
 #include "domain.h"
-#include "region.h"
-#include "input.h"
-#include "variable.h"
-#include "memory.h"
 #include "error.h"
-#include "utils.h"
-#include "fmt/format.h"
+#include "fix.h"
+#include "group.h"
+#include "input.h"
+#include "memory.h"
+#include "region.h"
+#include "update.h"
+#include "variable.h"
+
+#include <cstring>
+#include <vector>
 
 using namespace LAMMPS_NS;
 using namespace FixConst;
@@ -947,7 +947,7 @@ void Modify::add_fix(int narg, char **arg, int trysuffix)
 
 void Modify::add_fix(const std::string &fixcmd, int trysuffix)
 {
-  std::vector<std::string> args = utils::split_words(fixcmd);
+  auto args = utils::split_words(fixcmd);
   char **newarg = new char*[args.size()];
   int i=0;
   for (const auto &arg : args) {
@@ -1249,7 +1249,7 @@ void Modify::add_compute(int narg, char **arg, int trysuffix)
 
 void Modify::add_compute(const std::string &computecmd, int trysuffix)
 {
-  std::vector<std::string> args = utils::split_words(computecmd);
+  auto args = utils::split_words(computecmd);
   char **newarg = new char*[args.size()];
   int i=0;
   for (const auto &arg : args) {
diff --git a/src/modify.h b/src/modify.h
index 05eebc7e14..4c3db0a1b9 100644
--- a/src/modify.h
+++ b/src/modify.h
@@ -15,11 +15,14 @@
 #define LMP_MODIFY_H
 
 #include "pointers.h"
+
 #include <map>
-#include <string>
 
 namespace LAMMPS_NS {
 
+  class Compute;
+  class Fix;
+
 class Modify : protected Pointers {
   friend class Info;
   friend class FixSRP;
@@ -27,7 +30,6 @@ class Modify : protected Pointers {
   friend class RespaOMP;
 
  public:
-  int nfix,maxfix;
   int n_initial_integrate,n_post_integrate,n_pre_exchange;
   int n_pre_neighbor,n_post_neighbor;
   int n_pre_force,n_pre_reverse,n_post_force;
@@ -41,11 +43,12 @@ class Modify : protected Pointers {
   int nfix_restart_global;   // stored fix global info from restart file
   int nfix_restart_peratom;  // stored fix peratom info from restart file
 
-  class Fix **fix;           // list of fixes
+  int nfix,maxfix;
+  Fix **fix;                 // list of fixes
   int *fmask;                // bit mask for when each fix is applied
 
-  int ncompute,maxcompute;   // list of computes
-  class Compute **compute;
+  int ncompute,maxcompute;
+  Compute **compute;         // list of computes
 
   Modify(class LAMMPS *);
   virtual ~Modify();