adjust logic for choosing the communication cutoff to requests from @sjplimp

/* ----------------------------------------------------------------------

   determine suitable communication cutoff.

   it is the maximum of the user specified value and estimates based on

   the maximum neighbor list cutoff and largest bond equilibrium length.

   we use the 1.5x the bond equilibrium distance as cutoff, if only a

   bond style exists or only bond and angle styles exists.  If dihedrals

   or impropers are present we multiply by 2.0. This plus the

   "neighbor list skin" will become the default communication cutoff, if

   no pair style is defined and thus avoids all kinds of unexpected behavior

   for such systems.  If a pair style exists, the result is the maximum of

   the bond based cutoff and the largest pair cutoff and the user

   specified communication cutoff.

   use the larger of the user specified value and the cutoff required

   by the neighborlist build for pair styles.

   if bonded interactions exist, apply a heuristic based on the equilibrium

   bond length (use 1.5x r_bond if no dihedral or improper style exists

   otherwise 2x r_bond) plus neighbor list skin.

   if no user specified communication cutoff is given include this

   heuristic, otherwise ignore it.

   print a warning, if a user specified communication cutoff is overridden.

------------------------------------------------------------------------- */

double Comm::get_comm_cutoff()

{

  double maxcommcutoff = 0.0;

  double maxbondcutoff = 0.0;

  if (force->bond) {

    int n = atom->nbondtypes;

    for (int i = 1; i <= n; ++i)

      maxcommcutoff = MAX(maxcommcutoff,force->bond->equilibrium_distance(i));

      maxbondcutoff = MAX(maxbondcutoff,force->bond->equilibrium_distance(i));

    if (force->dihedral || force->improper) {

      maxcommcutoff *= 2.0;

      maxbondcutoff *= 2.0;

    } else {

      maxcommcutoff *=1.5;

      maxbondcutoff *=1.5;

    }

    maxcommcutoff += neighbor->skin;

    maxbondcutoff += neighbor->skin;

  }

  maxcommcutoff = MAX(maxcommcutoff,neighbor->cutneighmax);

  maxcommcutoff = MAX(maxcommcutoff,cutghostuser);

  if ((me == 0) && (cutghostuser > 0.0) && (maxcommcutoff > cutghostuser)) {

  // always take the larger of max neighbor list and user specified cutoff

  maxcommcutoff = MAX(cutghostuser,neighbor->cutneighmax);

  // consider cutoff estimate from bond length only if no user specified cutoff

  if (cutghostuser == 0.0) maxcommcutoff = MAX(maxcommcutoff,maxbondcutoff);

  // print warning if neighborlist cutoff overrides user cutoff or

  // applied bond based cutoff is larger than neighbor cutoff.

  if (me == 0) {

    if ( ((cutghostuser > 0.0) && (maxcommcutoff > cutghostuser))

         || ((cutghostuser == 0.0) && (maxcommcutoff == maxbondcutoff)) ) {

      char mesg[128];

    snprintf(mesg,128,"Communication cutoff is changed to %g",maxcommcutoff);

      snprintf(mesg,128,"Communication cutoff adjusted to %g",maxcommcutoff);

      error->warning(FLERR,mesg);

    }

  }

  return maxcommcutoff;

}