Journal of chemical theory and computation | 2019
Low-cost molecular excited states from a state-averaged resonating Hartree-Fock approach.
Abstract
Quantum chemistry methods that describe excited states on the same footing as the ground state are generally scarce. In previous work, Gill et al. (J. Phys. Chem. A 2008, 112, 13164) and later Sundstrom and Head-Gordon (J. Chem. Phys. 2014, 140, 114103) considered excited states resulting from a non-orthogonal configuration interaction (NOCI) on stationary solutions of the Hartree--Fock equations. We build upon those contributions and present the state-averaged resonating Hartree--Fock (sa-ResHF) method, which differs from NOCI in that spin-projection and orbital relaxation effects are incorporated from the onset. Our results in a set of small molecules (alanine, formaldehyde, acetaldehyde, acetone, formamide, and ethylene) suggest that sa-ResHF excitation energies are a notable improvement over configuration interaction singles (CIS), at a mean-field computational cost. The orbital relaxation in sa-ResHF, carried in the presence of a spin-projection operator, generally results in excitation energies that are closer to the EOM-CCSD and experimental values than the corresponding NOCI ones.