  We present a practical approach to treat static and dynamical correlation
accurately in large multi-configurational systems. The static correlation is
accounted for using the spin-flip approach which is well known for capturing
static correlation accurately at low-computational expense. Unlike previous
approaches to add dynamical correlation to spin-flip models which use
perturbation theory or coupled-cluster theory, we explore the ability to use
the on-top pair-density functional theory approaches recently developed by
Gagliardi and co-workers (JCTC, 10, 3669, 2014). External relaxations are
carried out in the spin-flip calculations though a restricted active space
framework for which a truncation scheme for the orbitals used in the external
excitation is presented. The performance of the approach is demonstrated by
computing energy gaps between ground and excited states for diradicals,
triradicals and linear polyacene chains ranging from naphthalene to dodecacene.
Accurate results are obtained using the new approach for these challenging
open-shell molecular systems.
