mcGFN-FF: an accurate force field for optimization and energetic screening of molecular crystals

This work presents a periodic extension of the GFN-FF force field for molecular crystals named mcGFN-FF. Non-covalent interactions in the force field are adjusted to reduce the systematic overbinding of the original, molecular version for molecular crystals. A diverse set of molecular crystal benchmarks for lattice energies and unit cell volumes is studied. The modified force field shows good results with a mean absolute relative deviation (MARD) of 19.9 % for lattice energies and 10.0 % for unit cell volumes. In many cases, mcGFN-FF approaches the accuracy of the GFN1-xTB quantum chemistry method which has an MARD of 18.7 % for lattice energies and 6.2 % for unit cell volumes. Further, the newly compiled mcVOL22 benchmark set is presented which features r2SCAN-D4/900 eV DFT reference volumes for molecular crystals with phosphorus-, sulfur-, and chlorine-containing compounds of various sizes. Overall, the mcGFN-FF poses an efficient tool for the optimization and energetic screening of molecular crystals containing elements up to radon.