update docs

Author: KuangYu

docs/user_guide/4.9ASE.md

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+# ASE MD Interface
+
+DMFF provides an interface for ASE, so the users can run MD using ASE. DMFF essentially provides an implementation of a DMFF calculator, which ASE can use to compute energy, force, and stress tensor.
+The basic usage is like:
+
+```python
+from dmff.mdtools.asetools import DMFFCalculator
+
+kwargs = {nonbondedMethod=app.PME,
+          ethresh=5e-4}
+atoms.calc = DMFFCalculator(pdb='init.pdb', ff_xml='ff.xml', rc=0.6, **kwargs)
+```
+In here, `rc` is the cutoff distance, which is used as `nonbondedCutoff` in potential function construction, and is also used to build the initial neighbor list for the initial compiling run of the potential function.
+the `kwargs` is directly passed to the `createPotential` function to generate the DMFF potential function, so the settings are the same as whatever you need in `createPotential` (except for `nonbondedCutoff`, which is obtained from `rc`).
+
+After the calculator is defined, the rest usage is the same as ASE. For users' convenience, we provide an example MD script in `examples/md_ase/run_md.py`. In this example, we use a water box with an EANN potential to show how a 
+structure optimization/NVE/NVT/NPT simulation can be conducted using ASE + DMFF.
+
+
+