diff --git a/src/USER-COLVARS/colvarproxy_lammps.cpp b/src/USER-COLVARS/colvarproxy_lammps.cpp
index 685c698ab6..2c43a3fb26 100644
--- a/src/USER-COLVARS/colvarproxy_lammps.cpp
+++ b/src/USER-COLVARS/colvarproxy_lammps.cpp
@@ -10,30 +10,19 @@
 
 #include "colvarproxy_lammps.h"
 #include <mpi.h>
-#include <sys/types.h>
 #include <sys/stat.h>
-#include <unistd.h>
-
 #include <cerrno>
-#include <cstdio>
 #include <cstring>
-
 #include <iostream>
-#include <sstream>
+#include <memory>
 #include <string>
 
 #include "lammps.h"
-#include "atom.h"
 #include "error.h"
 #include "output.h"
 #include "random_park.h"
 
-#include "fix_colvars.h"
-
 #include "colvarmodule.h"
-#include "colvar.h"
-#include "colvarbias.h"
-#include "colvaratoms.h"
 #include "colvarproxy.h"
 
 #define HASH_FAIL  -1
diff --git a/src/USER-COLVARS/colvarproxy_lammps.h b/src/USER-COLVARS/colvarproxy_lammps.h
index 3243a14fbd..8ed03309a3 100644
--- a/src/USER-COLVARS/colvarproxy_lammps.h
+++ b/src/USER-COLVARS/colvarproxy_lammps.h
@@ -10,39 +10,22 @@
 #ifndef COLVARPROXY_LAMMPS_H
 #define COLVARPROXY_LAMMPS_H
 
-#include "colvarproxy_lammps_version.h"
+#include "colvarproxy_lammps_version.h"  // IWYU pragma: export
+
+#include <mpi.h>
+#include <cstddef>
+#include <string>
+#include <vector>
 
 #include "colvarmodule.h"
 #include "colvarproxy.h"
-#include "colvarvalue.h"
+#include "colvartypes.h"
 
-#include "lammps.h"
-#include "domain.h"
-#include "force.h"
-#include "update.h"
-
-#include <string>
-#include <vector>
-#include <iostream>
-
-/* struct for packed data communication of coordinates and forces. */
-struct commdata {
-  int tag,type;
-  double x,y,z,m,q;
-};
-
-inline std::ostream & operator<< (std::ostream &out, const commdata &cd)
-{
-  out << " (" << cd.tag << "/" << cd.type << ": "
-      << cd.x << ", " << cd.y << ", " << cd.z << ") ";
-  return out;
-}
-
-// forward declarations
-namespace LAMMPS_NS {
-  class LAMMPS;
-  class RanPark;
-}
+#include "random_park.h"
+#include "lammps.h"  // IWYU pragma: keep
+#include "domain.h"  // IWYU pragma: keep
+#include "force.h"   // IWYU pragma: keep
+#include "update.h"  // IWYU pragma: keep
 
 /// \brief Communication between colvars and LAMMPS
 /// (implementation of \link colvarproxy \endlink)
diff --git a/src/USER-COLVARS/fix_colvars.cpp b/src/USER-COLVARS/fix_colvars.cpp
index 9331fe7abb..4a57e59fbd 100644
--- a/src/USER-COLVARS/fix_colvars.cpp
+++ b/src/USER-COLVARS/fix_colvars.cpp
@@ -25,26 +25,28 @@
 ------------------------------------------------------------------------- */
 
 #include "fix_colvars.h"
-#include <cmath>
-#include <cstdio>
+#include <mpi.h>
 #include <cstdlib>
 #include <cstring>
-#include <errno.h>
+#include <iostream>
+#include <string>
+#include <vector>
+#include <memory>
 
 #include "atom.h"
 #include "comm.h"
 #include "domain.h"
 #include "error.h"
-#include "group.h"
+#include "force.h"
 #include "memory.h"
 #include "modify.h"
-#include "random_park.h"
 #include "respa.h"
 #include "universe.h"
 #include "update.h"
 #include "citeme.h"
 
 #include "colvarproxy_lammps.h"
+#include "colvarmodule.h"
 
 static const char colvars_pub[] =
   "fix colvars command:\n\n"
@@ -57,6 +59,19 @@ static const char colvars_pub[] =
   " note =    {doi: 10.1080/00268976.2013.813594}\n"
   "}\n\n";
 
+/* struct for packed data communication of coordinates and forces. */
+struct LAMMPS_NS::commdata {
+  int tag,type;
+  double x,y,z,m,q;
+};
+
+inline std::ostream & operator<< (std::ostream &out, const LAMMPS_NS::commdata &cd)
+{
+  out << " (" << cd.tag << "/" << cd.type << ": "
+      << cd.x << ", " << cd.y << ", " << cd.z << ") ";
+  return out;
+}
+
 /* re-usable integer hash table code with static linkage. */
 
 /** hash table top level data structure */
diff --git a/src/USER-COLVARS/fix_colvars.h b/src/USER-COLVARS/fix_colvars.h
index a0c197fca4..985457e746 100644
--- a/src/USER-COLVARS/fix_colvars.h
+++ b/src/USER-COLVARS/fix_colvars.h
@@ -34,10 +34,9 @@ FixStyle(colvars,FixColvars)
 #define LMP_FIX_COLVARS_H
 
 #include "fix.h"
+#include <mpi.h>
 
-// forward declaration
 class colvarproxy_lammps;
-struct commdata;
 
 namespace LAMMPS_NS {
 
@@ -64,7 +63,7 @@ class FixColvars : public Fix {
   virtual void restart(char *);
 
  protected:
-  class colvarproxy_lammps *proxy; // pointer to the colvars proxy class
+  colvarproxy_lammps *proxy; // pointer to the colvars proxy class
   char *conf_file;     // name of colvars config file
   char *inp_name;      // name/prefix of colvars restart file
   char *out_name;      // prefix string for all output files
diff --git a/src/USER-COLVARS/group_ndx.cpp b/src/USER-COLVARS/group_ndx.cpp
index 7350712c40..1e37e2de9c 100644
--- a/src/USER-COLVARS/group_ndx.cpp
+++ b/src/USER-COLVARS/group_ndx.cpp
@@ -17,12 +17,11 @@
 ------------------------------------------------------------------------- */
 
 #include "group_ndx.h"
-#include <cstdio>
+#include <mpi.h>
 #include <cstdlib>
 #include "atom.h"
 #include "comm.h"
 #include "group.h"
-#include "memory.h"
 #include "error.h"
 
 using namespace LAMMPS_NS;
diff --git a/src/USER-COLVARS/ndx_group.cpp b/src/USER-COLVARS/ndx_group.cpp
index 5436ba7b6b..a1369df2fb 100644
--- a/src/USER-COLVARS/ndx_group.cpp
+++ b/src/USER-COLVARS/ndx_group.cpp
@@ -17,14 +17,12 @@
 ------------------------------------------------------------------------- */
 
 #include "ndx_group.h"
-#include <cstdio>
+#include <mpi.h>
 #include <cstdlib>
 #include <cstring>
 #include "atom.h"
 #include "comm.h"
 #include "group.h"
-#include "memory.h"
-#include "force.h"
 #include "error.h"
 
 using namespace LAMMPS_NS;