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.