Hybrid Electron-Proton Correlation in Multicomponent Density Functional Theory

In this contribution we investigate how far multicomponent DFT results can be improved by the admixture of Møller-Plesset perturbation theory electron-proton correlation energies. Three formulations are explored, based off the popular double-hybrid functionals B2PLYP, DSD-PBEP86 and PBEQIDH. Partial use of the PA23 proton binding affinities dataset is made to parameterize the ratio in the DFT:MP2 correlation energies. The resulting models are evaluated on a separate set of titratable molecules. The combination of NEO-DFT and NEO-MP2 electron-proton correlation leads up to a two-fold reduction in the RMSD compared to standard NEO-DFT, a trend that is confirmed in the independent test set. We apply the parameterized NEO-B2PLYP model to compute the energetics of protonated water hexamers as well as a challenging example for proton dynamics, a crown ether molecule. In the latter case we compare the energetics of localized vs shared proton configurations. Overall, a ratio of about 0.8:0.2 (DFT:MP2) in the electron-proton correlation delivers a robust improvement across the models, even with variations in the basis sets used and the type of chemical bonds investigated.