easyPARM: Automated, Versatile, and Reliable Force Field Parameters for Metal-Containing Molecules with Unique Labeling of Coordinating Atoms

The dynamics of metal centers are challenging to describe due to the vast variety of ligands, metals, and coordination spheres, hampering the existence of transferable force field parameters for classical molecular dynamics simulations. Here we present easyPARM, a python-based tool able to calculate force field parameters for a wide range of metal complexes from routine frequency calculations with electronic structure methods. The approach is based on the unique labeling strategy, in which each ligand atom that coordinates the metal receives a unique atom type. This design prevents parameters shortage, labeling duplication, and the necessity to post-process output files even for very complicated coordination spheres, whose parameterization process remain automatic. The program requires the Cartesian Hessian matrix, the geometry xyz file, and the atomic charges to provide reliable force field parameters extensively benchmarked against density functional theory dynamics both in the gas and condensed phases. The procedure allows the classical description of metal complexes at a low computational cost, with an accuracy as good as the quality of the Hessian matrix obtained by quantum chemistry methods. The tool is available free of charge in the GitHub platform (https://github.com/Abdelazim-Abdelgawwad/easyPARM.git).