Merge pull request brucefan1983#809 from zhyan0603/zhyan0603-GPUMD

add NEP_CPU link

Author: brucefan1983

README.md

@@ -43,6 +43,9 @@ This software is distributed under the GNU General Public License (GPL) version
 
 Various tools for `GPUMD` and `NEP` can be found in [tools](./tools/readme.md).
 
+## CPU version of NEP
+There is a standalone C++ implementation of the neuroevolution potential (NEP) in the [NEP_CPU repository](https://github.com/brucefan1983/NEP_CPU), which serves as the engine for many Python packages and provides an interface to the [LAMMPS package](https://github.com/lammps/lammps).
+
 ## Citations
 
 | Reference             | cite for what?                    |