Merge pull request #1830 from lammps/fix-bond-create-warn

add warning to fix bond/create

doc/src/Errors_warnings.rst

@@ -195,6 +195,12 @@ Doc page with :doc:`ERROR messages <Errors_messages>`
 *Fix SRD walls overlap but fix srd overlap not set*
    You likely want to set this in your input script.
 
+* Fix bond/create is used multiple times or with fix bond/break - may not work as expected*
+   When using fix bond/create multiple times or in combination with
+   fix bond/break, the individual fix instances do not share information
+   about changes they made at the same time step and thus it may result
+   in unexpected behavior.
+
 *Fix bond/swap will ignore defined angles*
    See the doc page for fix bond/swap for more info on this
    restriction.

doc/txt/Errors_warnings.txt

@@ -239,6 +239,13 @@ will be truncated to attempt to prevent the bond from blowing up. :dd
 
 You likely want to set this in your input script. :dd
 
+{ Fix bond/create is used multiple times or with fix bond/break - may not work as expected} :dt
+
+When using fix bond/create multiple times or in combination with
+fix bond/break, the individual fix instances do not share information
+about changes they made at the same time step and thus it may result
+in unexpected behavior. :dd
+
 {Fix bond/swap will ignore defined angles} :dt
 
 See the doc page for fix bond/swap for more info on this

src/MC/fix_bond_break.cpp

@@ -33,7 +33,8 @@ using namespace FixConst;
 
 FixBondBreak::FixBondBreak(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  partner(NULL), finalpartner(NULL), distsq(NULL), probability(NULL), broken(NULL), copy(NULL), random(NULL)
+  partner(NULL), finalpartner(NULL), distsq(NULL), probability(NULL), 
+  broken(NULL), copy(NULL), random(NULL)
 {
   if (narg < 6) error->all(FLERR,"Illegal fix bond/break command");
 

src/MC/fix_bond_create.cpp

@@ -18,6 +18,7 @@
 #include "respa.h"
 #include "atom.h"
 #include "force.h"
+#include "modify.h"
 #include "pair.h"
 #include "comm.h"
 #include "neighbor.h"
@@ -216,6 +217,19 @@ void FixBondCreate::init()
   if (force->pair == NULL || cutsq > force->pair->cutsq[iatomtype][jatomtype])
     error->all(FLERR,"Fix bond/create cutoff is longer than pairwise cutoff");
 
+  // warn if more than one fix bond/create or also a fix bond/break
+  // because this fix stores per-atom state in bondcount
+  //   if other fixes create/break bonds, this fix will not know about it
+
+  int count = 0;
+  for (int i = 0; i < modify->nfix; i++) {
+    if (strcmp(modify->fix[i]->style,"bond/create") == 0) count++;
+    if (strcmp(modify->fix[i]->style,"bond/break") == 0) count++;
+  }
+  if (count > 1 && me == 0) 
+    error->warning(FLERR,"Fix bond/create is used multiple times "
+                   " or with fix bond/break - may not work as expected");
+
   // enable angle/dihedral/improper creation if atype/dtype/itype
   //   option was used and a force field has been specified
 

src/MC/fix_bond_create.h

@@ -170,4 +170,11 @@ See the read_data command for info on setting the "extra special per
 atom" header value to allow for additional special values to be
 stored.
 
+W: Fix bond/create is used multiple times or with fix bond/break - may not work as expected
+
+When using fix bond/create multiple times or in combination with
+fix bond/break, the individual fix instances do not share information
+about changes they made at the same time step and thus it may result
+in unexpected behavior.
+
 */