add developer guide

README.md

@@ -33,8 +33,11 @@ This software is distributed under the GNU General Public License (GPL) version
 * You can also check other offline tutorials in the examples.
 
 ## Manual
-* Latest released version: https://gpumd.org/
-* Development version: https://gpumd.org/dev/
+* For users and developers:
+  * Latest released version: https://gpumd.org/
+  * Development version: https://gpumd.org/dev/
+* For developers only:
+  * [The developer guide](developers)
 
 ## Python packages related to GPUMD and/or NEP:
 

developers/readme.md

@@ -0,0 +1,77 @@
+# GPUMD Developer guide
+
+* This file contains information useful for existing and future `GPUMD` developers.
+
+* You can ignore this file if you are not interested in becoming a devoloper of `GPUMD`.
+
+* This is a work in progress, which will be constantly updated along with the GPUMD developement activities.
+
+## Using git to manage pull request (PR) contributions
+
+* If you are new to this, here is a good place to start reading: https://git-scm.com/book/ms/v2/GitHub-Contributing-to-a-Project
+
+## Development environments
+
+* You can develop `GPUMD` in either Linux or Windows, as long as you have a working CUDA and/or HIP development toolkit, and a suitable GPU (Nvidia or AMD).
+
+* GPUMD uses `make` to manage installation (or compilation).
+We have not seen the necessity of using `cmake`, yet.
+
+* There is no message passing interface (MPI) support in `GPUMD` yet, so currently you don't need to have MPI.
+We might add MPI support in the future.
+
+* To build `GPUMD`, simply type `make -f makefile` (for the CUDA version) or `make -f makefile.hip` (for the HIP version) and you will get the `gpumd` and `nep` binary files.
+
+## External dependencies
+
+* We make efforts to keep `GPUMD` as independent as possible.
+In principle, we only use CUDA C++ and HIP C++ in the source code.
+Particularly, we do not use Python in the source code.
+
+* If you want to introduce external dependence, the relevant code must be made optional, which will not be compiled by default.
+You alao need to give detailed instructions for setting up the necessary tools.
+
+* Currently we have two external dependencies, the `NetCDF` package and the `PLUMED` package.
+
+## Regression tests
+
+* These are a few regression tests in the `tests` folder.
+A developer should run the regression tests before starting a PR, saving the output files, and run the regression tests again after finishing the PR, making sure that the output files are identical.
+
+## Source files
+
+* All the source code of `GPUMD` can be found in the `src` folder.
+They are either a source file with the `.cu` extension or a header file with the `.cuh` extension. 
+
+* The source code was originally written in CUDA C++, but starting from GPUMD-v3.9.6, we support CUDA and HIP simultaneously.
+The CUDA and HIP specific APIs are collected in `src/utilities/gpu_macro.cuh`.
+If you use new CUDA and HIP APIs, they should be added to this file.
+
+* We use `clang-format` to format all the source and header files, according to the specifications in the file `.clang-format`, which can be found in the main folder of the `GPUMD` package.
+
+## Code structure
+
+* `main_nep`: Starting point for neuroevolution potential (NEP) training.
+
+* `main_gpumd`: Starting point for the `gpumd` executale. It contains the following modules:
+  * `model`: The model dealing with the model system in the simulation. This module is used by all the other modules under `main_gpumd`.
+  * `force`: The module contaning all the potentials.
+  * `integrate`: The module containing all the integrators/ensembles.
+  * `measure`: The module doing most of the on-the-fly calculations of the physical properties.
+  * `phonon`: The module for doing phonon calculations.
+  * `minimize`: The module for doing energy minimization.
+  * `mc`: The module for doing hybrid Monte Carlo and molecular dynamics (MCMD) simulations.
+* `utilities`: The module containing some common utilities used in many of other modules.
+
+## Units
+
+* Units for inputs and outputs should be specified in the user manual.
+
+* Internally, we use the fwollowing basic units:
+  * Energy: eV
+  * Legnth: Angstrom
+  * Mass: Dalton
+  * Temperature: K
+  * Charge: e (proton charge)
+
+