Localized Active Space Pair-Density Functional Theory

Accurate quantum chemical methods
for the prediction of spin-state
energy gaps for strongly correlated systems are computationally expensive
and scale poorly with the size of the system. This makes calculations
for many experimentally interesting molecules impractical even with
abundant computational resources. Previous work has shown that the
localized active space (LAS) self-consistent field (SCF) method can
be an efficient way to obtain multiconfiguration SCF wave functions
of comparable quality to the corresponding complete active space (CAS)
ones. To obtain quantitative results, a post-SCF method is needed
to estimate the complete correlation energy. One such method is multiconfiguration
pair-density functional theory (PDFT), which calculates the energy
based on the density and on-top pair density obtained from a multiconfiguration
wave function. In this work, we introduce localized-active-space PDFT,
which uses a LAS wave function for subsequent PDFT calculations. The
method is tested by computing spin-state energies and gaps in conjugated
organic molecules and a bimetallic compound and comparing to the corresponding
CAS-PDFT values.