# 3.9. Notes for building LAMMPS on Windows

## 3.9.1. General remarks

LAMMPS is developed and tested primarily on Linux machines. The vast majority of HPC clusters and supercomputers today run on Linux as well. While portability to other platforms is desired, it is not always achieved. The LAMMPS developers are dependent on LAMMPS users giving feedback and providing assistance in resolving portability issues. This is particularly true for compiling LAMMPS on Windows, since this platform has significant differences in some low-level functionality.

## 3.9.2. Running Linux on Windows

Before trying to build LAMMPS on Windows, please consider if the pre-compiled Windows binary packages are sufficient for your needs. If it is necessary for you to compile LAMMPS on a Windows machine (e.g. because it is your main desktop), please also consider using a virtual machine software and compile and run LAMMPS in a Linux virtual machine, or - if you have a sufficiently up-to-date Windows 10 installation - consider using the Windows subsystem for Linux. This optional Windows feature allows you to run the bash shell from Ubuntu from within Windows and from there on, you can pretty much use that shell like you are running on an Ubuntu Linux machine (e.g. installing software via apt-get and more). For more details on that, please see this tutorial.

## 3.9.3. Using a GNU GCC ported to Windows

One option for compiling LAMMPS on Windows natively that has been known to work in the past is to install a bash shell, unix shell utilities, perl, GNU make, and a GNU compiler ported to Windows. The Cygwin package provides a unix/linux interface to low-level Windows functions, so LAMMPS can be compiled on Windows. The necessary (minor) modifications to LAMMPS are included, but may not always up-to-date for recently added functionality and the corresponding new code. A machine makefile for using cygwin for the old build system is provided. Using CMake for this mode of compilation is untested and not likely to work.

When compiling for Windows do not set the -DLAMMPS_MEMALIGN define in the LMP_INC makefile variable and add -lwsock32 -lpsapi to the linker flags in LIB makefile variable. Try adding -static-libgcc or -static or both to the linker flags when your resulting LAMMPS Windows executable complains about missing .dll files. The CMake configuration should set this up automatically, but is untested.

In case of problems, you are recommended to contact somebody with experience in using Cygwin. If you do come across portability problems requiring changes to the LAMMPS source code, or figure out corrections yourself, please report them on the lammps-users mailing list, or file them as an issue or pull request on the LAMMPS GitHub project.

## 3.9.4. Using a cross-compiler

If you need to provide custom LAMMPS binaries for Windows, but do not need to do the compilation on Windows, please consider using a Linux to Windows cross-compiler. This is how currently the Windows binary packages are created by the LAMMPS developers. Because of that, this is probably the currently best tested and supported way to build LAMMPS executables for Windows. A CMake preset selecting all packages compatible with this cross-compilation build is provided. The GPU package can only be compiled with OpenCL support. To compile with MPI support, a pre-compiled library and the corresponding header files are required. When building with CMake the matching package will be downloaded automatically, but MPI support has to be explicitly enabled with -DBUILD_MPI=on.

Please keep in mind, though, that this only applies to compiling LAMMPS. Whether the resulting binaries do work correctly is rarely tested by the LAMMPS developers. We instead rely on the feedback of the users of these pre-compiled LAMMPS packages for Windows. We will try to resolve issues to the best of our abilities if we become aware of them. However this is subject to time constraints and focus on HPC platforms.

## 3.9.5. Native Visual C++ support

Support for the Visual C++ compilers is currently not available. The CMake build system is capable of creating suitable a Visual Studio style build environment, but the LAMMPS source code itself is not ported to fully support Visual C++. Volunteers to take on this task are welcome.