use simplified group->assign

{

  // wipe out all members if the group exists. gid==0 is group "all"

  int gid = group->find(name);

  if (gid > 0) {

    char *cmd[2];

    cmd[0] = name;

    cmd[1] = (char *)"clear";

    group->assign(2,cmd);

  }

  if (gid > 0) group->assign(std::string(name) + " clear");

  // map from global to local

  const int nlocal = atom->nlocal;