How do I cite ‘in.crack.lmp’? I’d like to use the methodology in a crack propagation study.
You don’t need to cite it. Just put a copy (with your modifications included) in the supporting information of your publication and then just state that it was taken from the LAMMPS source distribution. That itself you can cite with https://dx.doi.org/10.5281/zenodo.3726416 . Or just quote the URL to the specific LAMMPS release on GitHub. All releases are listed here: Releases · lammps/lammps · GitHub
Please also see: 1.8. Citing LAMMPS — LAMMPS documentation
Okay, got it. Thanks.