Generalized Active Space Pair Density Functional Theory: An Efficient Method to Study Large, Strongly Correlated, Conjugated Systems

Ghosh, S.; Cramer, C. J.; Truhlar, D. G.; Gagliardi, L.

* Chem. Sci.*
**2017**, *8*, 2741
(doi:10.1039/c6sc05036k).

Predicting ground- and excited-state properties of open-shell organic
molecules by electronic structure theory can be challenging because an
accurate treatment has to correctly describe both static and dynamic
electron correlation. Strongly correlated systems, i.e., systems with
near-degeneracy correlation effects, are particularly troublesome.
Multiconfigurational wave function methods based on an active space are
adequate in principle, but it is impractical to capture most of the dynamic
correlation in these methods for systems characterized by many active
electrons. We recently developed a new method called multiconfiguration
pair-density functional theory (MC-PDFT), that combines the advantages of
wave function theory and density functional theory to provide a more
practical treatment of strongly correlated systems. Here we present
calculations of the singlet–triplet gaps in oligoacenes ranging from
naphthalene to dodecacene. Calculations were performed for unprecedently
large orbitally optimized active spaces of 50 electrons in 50 orbitals, and
we test a range of active spaces and active space partitions, including
four kinds of frontier orbital partitions. We show that MC-PDFT can predict
the singlet-triplet splittings for oligoacenes consistent with the best
available and much more expensive methods, and indeed MC-PDFT may
constitute the benchmark against which those *other* models should be
compared, given the absence of experimental data.