support mapping to ghost type

doc/third-party/lammps-command.md

@@ -85,6 +85,7 @@ If the keyword `ttm` is set, electronic temperatures from [fix ttm command](http
 
 Only a single `pair_coeff` command is used with the deepmd style which specifies atom names. These are mapped to LAMMPS atom types (integers from 1 to Ntypes) by specifying Ntypes additional arguments after `* *` in the `pair_coeff` command.
 If atom names are not set in the `pair_coeff` command, the training parameter {ref}`type_map <model/type_map>` will be used by default.
+If a mapping value is specified as `NULL`, the mapping is not performed. This can be used when a deepmd potential is used as part of the hybrid pair style. The `NULL` values are placeholders for atom types that will be used with other potentials.
 If the training parameter {ref}`type_map <model/type_map>` is not set, atom names in the `pair_coeff` command cannot be set. In this case, atom type indexes in [`type.raw`](../data/system.md) (integers from 0 to Ntypes-1) will map to LAMMPS atom types.
 
 Spin is specified by keywords `virtual_len` and `spin_norm`. If the keyword `virtual_len` is set, the distance between virtual atom and its corresponding real atom for each type of magnetic atoms will be fed to the model as the spin parameters. If the keyword `spin_norm` is set, the magnitude of the magnetic moment for each type of magnetic atoms will be fed to the model as the spin parameters.

source/lmp/pair_deepmd.cpp

@@ -1153,6 +1153,10 @@ void PairDeepMD::coeff(int narg, char **arg) {
           break;
         }
       }
+      if (!found_element && "NULL" == type_name) {
+        type_idx_map.push_back(type_map.size());  // ghost type
+        found_element = true;
+      }
       if (!found_element) {
         error->all(FLERR, "pair_coeff: element " + type_name +
                               " not found in the model");