Valence ππ* Excitations in Benzene Studied by Multiconfiguration Pair-Density Functional Theory.

We explore the valence singlet and triplet ππ* excitations of benzene with complete active pace self-consistent field (CASSCF) theory, complete active space perturbation theory (CASPT2), and multiconfiguration pair-density functional theory (MC-PDFT) for four different choices of active space. We propose a new way to quantify the covalent and ionic character of the electronic states in terms of the components of the total electronic energy. We also explore the effect of scaling the exchange and correlation components of the on-top density functional used in MC-PDFT; we observe that increasing the exchange contribution improves the MC-PDFT excitation energies for benzene.