improve README

README.md

@@ -87,14 +87,16 @@ where $r_c$ is set by `r_max` and $N_L$ is set by `num_interactions` (MACE) / `n
 and rebuilds the neighbor list for ghost atoms.
 However, this approach is very inefficient.
 
-The alternative approach is to use the mapping passed from LAMMPS, which only does not support MPI.
+The alternative approach is to use the mapping passed from LAMMPS, which does not support MPI.
 One needs to set `DP_GNN_USE_MAPPING` when freezing the models,
 
 ```sh
 DP_GNN_USE_MAPPING=1 dp --pt freeze
 ```
 
 and request the mapping when using LAMMPS (also requires DeePMD-kit v3.0.0rc0 or above).
+By using the mapping, the ghost atoms will be mapped to the real atoms,
+so the regular neighbor list with a cutoff radius of $r_c$ can be used.
 
 ```lammps
 atom_modify map array