A New Mixing of Nonlocal Exchange and Nonlocal Correlation with Multiconfiguration Pair-Density Functional Theory.

We propose a hybrid multiconfiguration pair-density functional theory (HMC-PDFT) that is a weighted average of complete-active-space self-consistent-field (CASSCF) and multiconfiguration pair-density functional theory (MC-PDFT) energies with a semiempirical parameter to control the fraction of CASSCF energy. We also explore a more general two-parameter hybrid method with a scaled correlation energy that allows us to compare to the recently proposed λ-MC-PDFT method. We scan the parameter space for the scaled-correlation method using test sets consisting of electronic excitation energies and diatomic bond energies, and we find no significant improvement by introducing the scaling parameter. We find that unscaled HMC-PDFT offers significantly improved accuracy over both CASSCF and the original MC-PDFT for a wide range of systems, and we present as an example of this approach "tPBE0", the "translated" MC-PDFT generalization of the popular PBE0 hybrid Kohn-Sham density functional.