Spin-Flip Pair-Density Functional Theory: A Practical Approach To Treat Static and Dynamical Correlations in Large Molecules

We
present a practical approach to treat static and dynamical correlation
accurately in large multiconfigurational systems. The static correlation
is taken into account by 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 (J. Comput. Theor. Chem., 2014, 10, 3669). External relaxations are performed in the spin-flip calculations
through 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.