Removed misleading doc information and included errors

doc/src/Errors_messages.rst

@@ -1941,6 +1941,9 @@ Doc page with :doc:`WARNING messages <Errors_warnings>`
 *Compute ID for fix ave/time does not exist*
    Self-explanatory.
 
+*Compute ID for fix numdiff does not exist*
+   Self-explanatory.
+
 *Compute ID for fix store/state does not exist*
    Self-explanatory.
 
@@ -3779,6 +3782,14 @@ Doc page with :doc:`WARNING messages <Errors_warnings>`
    Self-explanatory.  The change in the box tilt is too extreme
    on a short timescale.
 
+*Fix numdiff requires an atom map, see atom_modify*
+   Self-explanatory. Efficient loop over all atoms for numerical
+   difference requires an atom map.
+
+*Fix numdiff requires consecutive atom IDs*
+   Self-explanatory. Efficient loop over all atoms for numerical
+   difference requires consecutive atom IDs.
+
 *Fix nve/asphere requires extended particles*
    This fix can only be used for particles with a shape setting.
 

doc/src/fix_numdiff.rst

@@ -53,8 +53,8 @@ by two times *Delta*.
    atom displacements that are used to generate finite difference
    approximations to the exact forces.  For typical systems, a value in
    the range 1e-xxx to 1e-yyy will probably work well.  However, the
-   best value will depend on a multitude of factors including the LAMMPS
-   unit style, the stiffness of the interatomic potential,the
+   best value will depend on a multitude of factors including 
+   the stiffness of the interatomic potential,the
    thermodynamic state of the material being probed, and so on. The only
    way to be sure that you have made a good choice is to do a
    sensitivity study on a representative atomic configuration, sweeping 

src/fix_numdiff.cpp

@@ -73,7 +73,7 @@ FixNumDiff::FixNumDiff(LAMMPS *lmp, int narg, char **arg) :
   temp_f = NULL;
 
   if (atom->map_style == 0)
-    error->all(FLERR,"Fix_numdiff requires an atom map, see atom_modify");
+    error->all(FLERR,"Fix numdiff requires an atom map, see atom_modify");
 
   // perform initial allocation of atom-based arrays
   // zero numdiff_forces since dump may access it on timestep 0

src/fix_numdiff.h

@@ -73,9 +73,15 @@ Self-explanatory.  Check the input script syntax and compare to the
 documentation for the command.  You can use -echo screen as a
 command-line option when running LAMMPS to see the offending line.
 
-E: Fix_numdiff requires an atom map, see atom_modify
+E: Fix numdiff requires an atom map, see atom_modify
 
-Self-explanatory.
+Self-explanatory. Efficient loop over all atoms for numerical difference
+requires an atom map.
+
+E: Fix numdiff requires consecutive atom IDs
+
+Self-explanatory. Efficient loop over all atoms for numerical difference
+requires consecutive atom IDs.
 
 E: Compute ID for fix numdiff does not exist