better argument checking with threebody off, disallow invalid uses

this now reads the potential file only on the first pair_coeff command
and also creates the element to atom type map then. all following
pair_coeff commands must be consistent. LAMMPS will stop if not.

we also need to explicity assign setflag and must not have it reset
when creating the mappings.

doc/src/pair_sw.rst

@@ -176,9 +176,13 @@ are placeholders for atom types that will be used with other potentials.
 
 .. note::
 
-   When the *threebody off* keyword is used, multiple pair_coeff commands may
-   be used to specific the pairs of atoms which don't require three-body term.
-   In these cases, the first 2 arguments are not required to be \* \*.
+   When the *threebody off* keyword is used, multiple pair_coeff
+   commands may be used to specific the pairs of atoms which don't
+   require three-body term.  In these cases, the first 2 arguments are
+   not required to be \* \*, the potential parameter file is only read
+   by the first :doc:`pair_coeff <command>` and the following element to
+   atom type mappings must be consistent across all *pair_coeff*
+   statements.
 
 Stillinger-Weber files in the *potentials* directory of the LAMMPS
 distribution have a ".sw" suffix.  Lines that are not blank or

src/MANYBODY/pair_sw.cpp

@@ -46,6 +46,7 @@ PairSW::PairSW(LAMMPS *lmp) : Pair(lmp)
   centroidstressflag = CENTROID_NOTAVAIL;
   unit_convert_flag = utils::get_supported_conversions(utils::ENERGY);
   skip_threebody_flag = false;
+  params_mapped = 0;
 
   params = nullptr;
 
@@ -225,12 +226,12 @@ void PairSW::compute(int eflag, int vflag)
 void PairSW::allocate()
 {
   allocated = 1;
-  int n = atom->ntypes;
+  int np1 = atom->ntypes + 1;
 
-  memory->create(setflag,n+1,n+1,"pair:setflag");
-  memory->create(cutsq,n+1,n+1,"pair:cutsq");
-  memory->create(neighshort,maxshort,"pair:neighshort");
-  map = new int[n+1];
+  memory->create(setflag, np1, np1, "pair:setflag");
+  memory->create(cutsq, np1, np1, "pair:cutsq");
+  memory->create(neighshort, maxshort, "pair:neighshort");
+  map = new int[np1];
 }
 
 /* ----------------------------------------------------------------------
@@ -262,12 +263,39 @@ void PairSW::coeff(int narg, char **arg)
 {
   if (!allocated) allocate();
 
-  map_element2type(narg-3,arg+3);
+  // read potential file and set up element maps only once
+  if (one_coeff || !params_mapped) {
+    map_element2type(narg-3, arg+3, (one_coeff != 0));
 
     // read potential file and initialize potential parameters
 
     read_file(arg[2]);
     setup_params();
+    params_mapped = 1;
+  }
+
+  if (!one_coeff) {
+    int ilo, ihi, jlo, jhi;
+    utils::bounds(FLERR, arg[0], 1, atom->ntypes, ilo, ihi, error);
+    utils::bounds(FLERR, arg[1], 1, atom->ntypes, jlo, jhi, error);
+
+    int count = 0;
+    for (int i = ilo; i <= ihi; i++) {
+      if (((map[i] >= 0) && (strcmp(arg[i+2], elements[map[i]]) != 0)) ||
+          ((map[i] < 0) && (strcmp(arg[i+2], "NULL") != 0)))
+        error->all(FLERR, "Must use consistent type to element mappings with threebody off");
+      if (map[i] < 0) error->all(FLERR, "Must not set pair_coeff mapped to NULL element");
+      for (int j = MAX(jlo, i); j <= jhi; j++) {
+        if (((map[j] >= 0) && (strcmp(arg[j+2], elements[map[j]]) != 0)) ||
+            ((map[j] < 0) && (strcmp(arg[j+2], "NULL") != 0)))
+          error->all(FLERR, "Must use consistent type to element mappings with threebody off");
+        if (map[j] < 0) error->all(FLERR, "Must not set pair_coeff mapped to NULL element");
+        setflag[i][j] = 1;
+        count++;
+      }
+    }
+    if (count == 0) error->all(FLERR, "Incorrect args for pair coefficients");
+  }
 }
 
 /* ----------------------------------------------------------------------

src/MANYBODY/pair_sw.h

@@ -54,6 +54,7 @@ class PairSW : public Pair {
   int maxshort;               // size of short neighbor list array
   int *neighshort;            // short neighbor list array
   int skip_threebody_flag;    // whether to run threebody loop
+  int params_mapped;          // whether parameters have been read and mapped to elements
 
   void settings(int, char **) override;
   virtual void allocate();