diff --git a/src/SHOCK/fix_append_atoms.cpp b/src/SHOCK/fix_append_atoms.cpp
index b77067b939..b2bd280dbf 100644
--- a/src/SHOCK/fix_append_atoms.cpp
+++ b/src/SHOCK/fix_append_atoms.cpp
@@ -70,7 +70,7 @@ FixAppendAtoms::FixAppendAtoms(LAMMPS *lmp, int narg, char **arg) :
       if (domain->boundary[0][0] != 3) 
 	error->all(FLERR,"Append boundary must be shrink/minimum");
     } else if (strcmp(arg[iarg],"xhi") == 0) {
-      error->all(FLERR,"Only zhi currently implemented for append_atom");
+      error->all(FLERR,"Only zhi currently implemented for append_atoms");
       xhiflag = 1;
       iarg++;
       if (domain->boundary[0][1] != 3) 
diff --git a/src/SHOCK/fix_append_atoms.h b/src/SHOCK/fix_append_atoms.h
index cdd9428405..77fb03fd27 100644
--- a/src/SHOCK/fix_append_atoms.h
+++ b/src/SHOCK/fix_append_atoms.h
@@ -65,39 +65,40 @@ command-line option when running LAMMPS to see the offending line.
 
 E: Only zhi currently implemented for append_atoms
 
-UNDOCUMENTED
+The face keyword must be zhi.
 
 E: Append boundary must be shrink/minimum
 
-UNDOCUMENTED
-
-E: Only zhi currently implemented for append_atom
-
-UNDOCUMENTED
+The boundary style of the face where atoms are added
+must be of type m (shrink/minimum). 
 
 E: Bad fix ID in fix append_atoms command
 
-UNDOCUMENTED
+The value of the fix_id for keyword spatial
+must start with the suffix f_.  
 
 E: Cannot use append_atoms in periodic dimension
 
-UNDOCUMENTED
+The boundary style of the face where atoms are added
+can not be of type p (periodic). 
 
 E: Cannot append atoms to a triclinic box
 
-UNDOCUMENTED
+The simulation box must be defined with edges alligned
+with the Cartesian axes.
 
 E: Use of fix append_atoms with undefined lattice
 
-UNDOCUMENTED
+A lattice must be defined before using this fix.
 
 E: Fix ID for fix ave/spatial does not exist
 
-Self-explanatory.
+The fix_id value for the spatial keyword does
+not correspond to any defined fix.
 
 E: Must define lattice to append_atoms
 
-UNDOCUMENTED
+A lattice must be defined before using this fix.
 
 U: must define lattice to append_atoms
 
diff --git a/src/SHOCK/fix_nphug.cpp b/src/SHOCK/fix_nphug.cpp
index 746498915b..ed3c8f74e2 100644
--- a/src/SHOCK/fix_nphug.cpp
+++ b/src/SHOCK/fix_nphug.cpp
@@ -61,7 +61,7 @@ FixNPHug::FixNPHug(LAMMPS *lmp, int narg, char **arg) :
   if (p_start[0] != p_stop[0] ||
       p_start[1] != p_stop[1] ||  
       p_start[2] != p_stop[2])
-    error->all(FLERR,"Invalid argument for fix nphug");
+    error->all(FLERR,"Pstart and Pstop must have the same value");
 
   // uniaxial = 0 means hydrostatic compression
   // uniaxial = 1 means uniaxial compression
@@ -97,7 +97,7 @@ FixNPHug::FixNPHug(LAMMPS *lmp, int narg, char **arg) :
       uniaxial = 1;
       idir = 2;
 
-    } else error->all(FLERR,"Invalid argument for fix nphug");
+    } else error->all(FLERR,"Specified target stress must be uniaxial or hydrostatic");
 
     // triclinic hydrostatic compression
 
@@ -110,7 +110,7 @@ FixNPHug::FixNPHug(LAMMPS *lmp, int narg, char **arg) :
 	p_start[5] == 0.0 )
       uniaxial = 0;
 
-    else error->all(FLERR,"Invalid argument for fix nphug");
+    else error->all(FLERR,"For triclinic deformation, specified target stress must be hydrostatic");
   }
 
   if (!tstat_flag)
diff --git a/src/SHOCK/fix_nphug.h b/src/SHOCK/fix_nphug.h
index 0cf9ea0c21..0ca169885e 100644
--- a/src/SHOCK/fix_nphug.h
+++ b/src/SHOCK/fix_nphug.h
@@ -62,21 +62,30 @@ class FixNPHug : public FixNH {
 
 /* ERROR/WARNING messages:
 
-E: Invalid argument for fix nphug
+E: Pstart and Pstop must have the same value
 
-UNDOCUMENTED
+Self-explanatory
+
+E: Specified target stress must be uniaxial or hydrostatic
+
+Self-explanatory
+
+E: For triclinic deformation, specified target stress must be hydrostatic
+
+Triclinic pressure control is allowed using the tri keyword, but
+non-hydrostatic pressure control can not be used in this case.
 
 E: Temperature control must be used with fix nphug
 
-UNDOCUMENTED
+The temp keyword must be provided.
 
 E: Pressure control must be used with fix nphug
 
-UNDOCUMENTED
+A pressure control keyword (iso, aniso, tri, x, y, or z) must be provided.
 
 E: Potential energy ID for fix nvt/nph/npt does not exist
 
-UNDOCUMENTED
+A compute for potential energy must be defined.
 
 E: Illegal ... command
 
diff --git a/src/SHOCK/fix_wall_piston.cpp b/src/SHOCK/fix_wall_piston.cpp
index f9fc94c020..dc78fa5f03 100644
--- a/src/SHOCK/fix_wall_piston.cpp
+++ b/src/SHOCK/fix_wall_piston.cpp
@@ -79,7 +79,7 @@ FixWallPiston::FixWallPiston(LAMMPS *lmp, int narg, char **arg) :
       z0 = atof(arg[iarg+3]);
       iarg += 4;
     } else if (strcmp(arg[iarg],"temp") == 0) {
-      if (iarg+5 > narg) error->all(FLERR,"Illegal fix wall/pistons command");
+      if (iarg+5 > narg) error->all(FLERR,"Illegal fix wall/piston command");
       tempflag = 1;
       t_target = atof(arg[iarg+1]);
       t_period = atof(arg[iarg+2]);
@@ -88,7 +88,7 @@ FixWallPiston::FixWallPiston(LAMMPS *lmp, int narg, char **arg) :
       if (t_target <= 0) error->all(FLERR,"Illegal fix wall/piston command");
       if (t_period <= 0) error->all(FLERR,"Illegal fix wall/piston command");
       if (t_extent <= 0) error->all(FLERR,"Illegal fix wall/piston command");
-      if (tseed <= 0) error->all(FLERR,"Illegal fix wall/pistons command");
+      if (tseed <= 0) error->all(FLERR,"Illegal fix wall/piston command");
       randomt = new RanMars(lmp,tseed + comm->me);
       gfactor1 = new double[atom->ntypes+1];
       gfactor2 = new double[atom->ntypes+1];
diff --git a/src/SHOCK/fix_wall_piston.h b/src/SHOCK/fix_wall_piston.h
index 67d9827a3f..f7f108a9a6 100644
--- a/src/SHOCK/fix_wall_piston.h
+++ b/src/SHOCK/fix_wall_piston.h
@@ -55,26 +55,29 @@ command-line option when running LAMMPS to see the offending line.
 
 E: Fix wall/piston command only available at zlo
 
-UNDOCUMENTED
+The face keyword must be zlo.
 
 E: Must shrink-wrap piston boundary
 
-UNDOCUMENTED
+The boundary style of the face where the piston is
+applied must be of type s (shrink-wrapped). 
 
 E: Illegal fix wall/piston velocity
 
-UNDOCUMENTED
+The piston velocity must be positive.
 
 E: Cannot use wall in periodic dimension
 
-Self-explanatory.
+The boundary style of the face where atoms are added
+can not be of type p (periodic). 
 
 E: Use of fix wall/piston with undefined lattice
 
-UNDOCUMENTED
+A lattice must be defined before using this fix.
 
 E: NL ramp in wall/piston only implemented in zlo for now
 
-UNDOCUMENTED
+The ramp keyword can only be used for piston applied
+to face zlo.
 
 */