Extension of the fragment molecular orbital method to treat large open-shell systems in solution

Abstract The analytic energy gradient is developed for open-shell fragment molecular orbital methods (FMO) interfaced with the polarizable continuum model. A pair interaction energy decomposition analysis, spin density and atomic spin populations are also developed. The accuracy is tested on a polypeptide oxidation reaction in solution, comparing three isomers. FMO is applied to study the spin density distribution and radical stabilization of the doublet state of the ribonucleotide reductase in solution, containing 11 536 atoms.