add group style to create an empty group directly

doc/src/group.txt

@@ -13,11 +13,13 @@ group command :h3
 group ID style args :pre
 
 ID = user-defined name of the group :ulb,l
-style = {delete} or {region} or {type} or {id} or {molecule} or {variable} or \
+style = {delete} or {clear} or {empty} or {region} or \
+        {type} or {id} or {molecule} or {variable} or \
         {include} or {subtract} or {union} or {intersect} or \
         {dynamic} or {static} :l
   {delete} = no args
   {clear} = no args
+  {empty} = no args
   {region} args = region-ID
   {type} or {id} or {molecule}
     args = list of one or more atom types, atom IDs, or molecule IDs
@@ -99,6 +101,10 @@ e.g. using the "run every"_run.html command if a fix or compute or
 other operation expects the atoms in the group to remain constant, but
 LAMMPS does not check for this.
 
+The {empty} style creates an empty group, which is useful for commands
+like "fix gcmc"_fix_gcmc.html or with complex scripts that add atoms
+to a group.
+
 The {region} style puts all atoms in the region volume into the group.
 Note that this is a static one-time assignment.  The atoms remain
 assigned (or not assigned) to the group even in they later move out of

src/group.cpp

@@ -188,6 +188,12 @@ void Group::assign(int narg, char **arg)
       if (domain->regions[iregion]->match(x[i][0],x[i][1],x[i][2]))
         mask[i] |= bit;
 
+  // create an empty group
+
+  } else if (strcmp(arg[1],"empty") == 0) {
+
+    ; // nothing to do here
+
   // style = type, molecule, id
   // add to group if atom matches type/molecule/id or condition