A Combined Energy-Force Fitting Procedure to Develop DFT-Based Force Fields

A general method to develop accurate force fields from density functional theory (DFT) computations in periodic systems is here presented. The novelty of the method consists of the inclusion of both potential energy and forces in the same fit, by using an automated procedure to balance the relative weight of the two quantities. A thorough analysis of the method capabilities is carried out by modeling the dispersion interactions of argon adsorbed in ZIF-8. While a pure energy fit leaves the parameters of some atoms kinds underdetermined, a pure forces fit gives well converged results but fails in properly reproducing the potential energy of the system. The optimal solution is found when a small contribution of forces is included in the energy fit: this allows one to fuse together the best features of the two fits, giving converged results and good reproduction of both energy and forces. The force field parameters for various DFT functionals, namely, DFT-D2, DFT-D3, vdW-DF2, and rVV10, are derived, and the corresponding isotherms are compared to experimental values. For the system here investigated, the best agreement with experiments is found for the DFT-D2 functional.