Add doc for model deviation (#1153)

* Add .idea to .gitignore

* Add doc for models in model deviation.

* Update doc/third-party/lammps-command.md

Co-authored-by: Jinzhe Zeng <[email protected]>

Co-authored-by: Han Wang <[email protected]>
Co-authored-by: Jinzhe Zeng <[email protected]>

Author: 3 people

.gitignore

@@ -30,5 +30,6 @@ API_CC
 doc/api_py/
 dp/
 build_lammps/
+.idea/
 build_tests/
 build_cc_tests

doc/third-party/lammps-command.md

@@ -18,7 +18,9 @@ The DeePMD-kit package provides the pair_style `deepmd`
 pair_style deepmd models ... keyword value ...
 ```
 - deepmd = style of this pair_style
-- models = frozen model(s) to compute the interaction. If multiple models are provided, then the model deviation will be computed
+- models = frozen model(s) to compute the interaction. 
+If multiple models are provided, then only the first model serves to provide energy and force prediction for each timestep of molecular dynamics, 
+and the model deviation will be computed among all models every `out_freq` timesteps.
 - keyword = *out_file* or *out_freq* or *fparam* or *atomic* or *relative*
 <pre>
     <i>out_file</i> value = filename