diff --git a/src/EXTRA-FIX/fix_ttm.cpp b/src/EXTRA-FIX/fix_ttm.cpp
index 13fbd4aff4..4c0d2e9cd5 100644
--- a/src/EXTRA-FIX/fix_ttm.cpp
+++ b/src/EXTRA-FIX/fix_ttm.cpp
@@ -46,6 +46,18 @@ using namespace FixConst;
 static constexpr int OFFSET = 16384;
 static constexpr double SHIFT = 0.0;
 
+// helper class
+
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
+
 /* ---------------------------------------------------------------------- */
 
 FixTTM::FixTTM(LAMMPS *lmp, int narg, char **arg) :
diff --git a/src/EXTRA-FIX/fix_ttm.h b/src/EXTRA-FIX/fix_ttm.h
index 1a2e9fb24a..da782b1f5a 100644
--- a/src/EXTRA-FIX/fix_ttm.h
+++ b/src/EXTRA-FIX/fix_ttm.h
@@ -77,14 +77,6 @@ class FixTTM : public Fix {
   virtual void deallocate_grid();
   virtual void read_electron_temperatures(const std::string &);
   virtual void write_electron_temperatures(const std::string &);
-
-  class parser_error : public std::exception {
-    std::string message;
-
-   public:
-    parser_error(const std::string &mesg) { message = mesg; }
-    const char *what() const noexcept { return message.c_str(); }
-  };
 };
 
 }    // namespace LAMMPS_NS
diff --git a/src/EXTRA-FIX/fix_ttm_grid.cpp b/src/EXTRA-FIX/fix_ttm_grid.cpp
index 7b3f434907..b42b876a3b 100644
--- a/src/EXTRA-FIX/fix_ttm_grid.cpp
+++ b/src/EXTRA-FIX/fix_ttm_grid.cpp
@@ -41,6 +41,18 @@ static constexpr int MAXLINE = 256;
 static constexpr int CHUNK = 1024;
 static constexpr int OFFSET = 16384;
 
+// helper class
+
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
+
 /* ---------------------------------------------------------------------- */
 
 FixTTMGrid::FixTTMGrid(LAMMPS *lmp, int narg, char **arg) :
diff --git a/src/EXTRA-FIX/fix_ttm_mod.cpp b/src/EXTRA-FIX/fix_ttm_mod.cpp
index 0aef2ec4ff..d39d50c9e5 100644
--- a/src/EXTRA-FIX/fix_ttm_mod.cpp
+++ b/src/EXTRA-FIX/fix_ttm_mod.cpp
@@ -41,6 +41,18 @@ using namespace LAMMPS_NS;
 using namespace FixConst;
 using namespace MathConst;
 
+// helper class
+
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
+
 // OFFSET avoids outside-of-box atoms being rounded to grid pts incorrectly
 // SHIFT = 0.0 assigns atoms to lower-left grid pt
 // SHIFT = 0.5 assigns atoms to nearest grid pt
diff --git a/src/EXTRA-FIX/fix_ttm_mod.h b/src/EXTRA-FIX/fix_ttm_mod.h
index f26f270e7a..21eb0efcaa 100644
--- a/src/EXTRA-FIX/fix_ttm_mod.h
+++ b/src/EXTRA-FIX/fix_ttm_mod.h
@@ -87,14 +87,6 @@ class FixTTMMod : public Fix {
   void read_parameters(const std::string &);
   void read_electron_temperatures(const std::string &);
   void write_electron_temperatures(const std::string &);
-
-  class parser_error : public std::exception {
-    std::string message;
-
-   public:
-    parser_error(const std::string &mesg) { message = mesg; }
-    const char *what() const noexcept { return message.c_str(); }
-  };
 };
 
 }    // namespace LAMMPS_NS
diff --git a/src/QEQ/fix_qeq.cpp b/src/QEQ/fix_qeq.cpp
index 79205f7c81..b43afb00d6 100644
--- a/src/QEQ/fix_qeq.cpp
+++ b/src/QEQ/fix_qeq.cpp
@@ -42,12 +42,14 @@ using namespace FixConst;
 
 #define MAXLINE 1024
 
-class parser_error : public std::exception {
-  std::string message;
-public:
-  parser_error(const std::string &mesg) { message = mesg; }
-  const char *what() const noexcept { return message.c_str(); }
-};
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
 
 /* ---------------------------------------------------------------------- */
 
diff --git a/src/REAXFF/fix_acks2_reaxff.cpp b/src/REAXFF/fix_acks2_reaxff.cpp
index f19b31f9ed..ec14ed72b2 100644
--- a/src/REAXFF/fix_acks2_reaxff.cpp
+++ b/src/REAXFF/fix_acks2_reaxff.cpp
@@ -43,12 +43,14 @@
 using namespace LAMMPS_NS;
 using namespace FixConst;
 
-class parser_error : public std::exception {
-  std::string message;
-public:
-  parser_error(const std::string &mesg) { message = mesg; }
-  const char *what() const noexcept { return message.c_str(); }
-};
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
 
 static const char cite_fix_acks2_reax[] =
   "fix acks2/reaxff command:\n\n"
diff --git a/src/REAXFF/fix_qeq_reaxff.cpp b/src/REAXFF/fix_qeq_reaxff.cpp
index f0ba651a26..89ee3f338f 100644
--- a/src/REAXFF/fix_qeq_reaxff.cpp
+++ b/src/REAXFF/fix_qeq_reaxff.cpp
@@ -49,12 +49,14 @@
 using namespace LAMMPS_NS;
 using namespace FixConst;
 
-class parser_error : public std::exception {
-  std::string message;
-public:
-  parser_error(const std::string &mesg) { message = mesg; }
-  const char *what() const noexcept { return message.c_str(); }
-};
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
 
 static constexpr double EV_TO_KCAL_PER_MOL = 14.4;
 static constexpr double SMALL = 1.0e-14;
diff --git a/src/REAXFF/reaxff_control.cpp b/src/REAXFF/reaxff_control.cpp
index 2020e8a5c3..90f12b2031 100644
--- a/src/REAXFF/reaxff_control.cpp
+++ b/src/REAXFF/reaxff_control.cpp
@@ -40,6 +40,15 @@ using LAMMPS_NS::utils::sfgets;
 using LAMMPS_NS::utils::logmesg;
 using LAMMPS_NS::ValueTokenizer;
 
+namespace {
+  class parser_error : public std::exception {
+    std::string message;
+  public:
+    parser_error(const std::string &mesg) { message = mesg; }
+    const char *what() const noexcept { return message.c_str(); }
+  };
+}
+
 namespace ReaxFF {
   static std::unordered_set<std::string> inactive_keywords = {
     "ensemble_type", "nsteps", "dt", "proc_by_dim", "random_vel",
@@ -54,13 +63,6 @@ namespace ReaxFF {
     "energy_update_freq", "atom_info", "atom_velocities", "atom_forces",
     "bond_info", "angle_info" };
 
-  class parser_error : public std::exception {
-    std::string message;
-  public:
-    parser_error(const std::string &mesg) { message = mesg; }
-    const char *what() const noexcept { return message.c_str(); }
-  };
-
   // NOTE: this function is run on MPI rank 0 only
 
   void Read_Control_File(const char *control_file, control_params *control)
diff --git a/src/REAXFF/reaxff_ffield.cpp b/src/REAXFF/reaxff_ffield.cpp
index 605e077724..db4a63972e 100644
--- a/src/REAXFF/reaxff_ffield.cpp
+++ b/src/REAXFF/reaxff_ffield.cpp
@@ -41,14 +41,16 @@ using LAMMPS_NS::utils::open_potential;
 using LAMMPS_NS::utils::getsyserror;
 using LAMMPS_NS::utils::uppercase;
 
-namespace ReaxFF {
-
+namespace {
   class parser_error : public std::exception {
     std::string message;
   public:
     parser_error(const std::string &mesg) { message = mesg; }
     const char *what() const noexcept { return message.c_str(); }
   };
+}
+
+namespace ReaxFF {
 
   void Read_Force_Field(const char *filename, reax_interaction *reax,
                         control_params *control, MPI_Comm world)