State-interaction pair-density functional theory.

The accurate description of ground- and excited-state potential energy surfaces poses a challenge for many electronic structure methods, especially in regions where strong electronic state interaction occurs. Here we introduce a new methodology, state-interaction pair-density functional theory (SI-PDFT), to target molecular systems exhibiting strong interaction of electronic states. SI-PDFT is an extension of multiconfiguration pair-density functional theory in which a set of N electronic states is generated through the diagonalization of an N × N effective Hamiltonian. We demonstrate the accuracy of the method by performing calculations on the ionic-neutral avoided crossing in lithium fluoride and the 1ππ-1πσ* avoided crossing in the H-O bond photodissociation in phenol. We show that SI-PDFT can be a useful tool in the study of photochemistry and nonadiabatic dynamics.