Add docs in README for deepmd/npy/mixed

README.md

@@ -62,6 +62,8 @@ The `System` or `LabeledSystem` can be constructed from the following file forma
 | deepmd  | npy    | True         | False   | System        | 'deepmd/npy'  |
 | deepmd  | raw    | True         | True    | LabeledSystem | 'deepmd/raw'  |
 | deepmd  | npy    | True         | True    | LabeledSystem | 'deepmd/npy'  |
+| deepmd  | npy    | True         | True    | MultiSystems | 'deepmd/npy/mixed'  |
+| deepmd  | npy    | True         | False    | MultiSystems | 'deepmd/npy/mixed'  |
 | gaussian| log    | False        | True    | LabeledSystem | 'gaussian/log'|
 | gaussian| log    | True         | True    | LabeledSystem | 'gaussian/md' |
 | siesta  | output | False        | True    | LabeledSystem | 'siesta/output'|
@@ -278,6 +280,28 @@ print(syst.get_charge())  # return the total charge of the system
 
 If a valence of 3 is detected on carbon, the formal charge will be assigned to -1. Because for most cases (in alkynyl anion, isonitrile, cyclopentadienyl anion), the formal charge on 3-valence carbon is -1, and this is also consisent with the 8-electron rule.
 
+## Mixed Type Format
+The format `deepmd/npy/mixed` is the mixed type numpy format for DeePMD-kit, and can be loaded or dumped through class `dpdata.MultiSystems`.
+
+Under this format, systems with the same number of atoms but different formula can be put together
+for a larger system, especially when the frame numbers in systems are sparse.
+
+This also helps to mixture the type information together for model training with type embedding network.
+
+Here are examples using `deepmd/npy/mixed` format:
+
+- Dump a MultiSystems into a mixed type numpy directory:
+```python
+import dpdata
+dpdata.MultiSystems(*systems).to_deepmd_npy_mixed("mixed_dir")
+```
+
+- Load a mixed type data into a MultiSystems:
+```python
+import dpdata
+dpdata.MultiSystems().load_systems_from_file("mixed_dir", fmt="deepmd/npy/mixed")
+```
+
 # Plugins
 
 One can follow [a simple example](plugin_example/) to add their own format by creating and installing plugins. It's critical to add the [Format](dpdata/format.py) class to `entry_points['dpdata.plugins']` in [`pyproject.toml`](plugin_example/pyproject.toml):