title: Communication: Adjusting charge transfer state energies for configuration interaction singles: without any parameterization and with minimal cost.
abstract: In a recent article, we showed that configuration interaction singles (CIS) has a systematic bias against charge-transfer (CT) states: CT vertical excitation energies are consistently too high (by 1-2 eV) as compared with non-CT energies [J. E. Subotnik, J. Chem. Phys. 137, 071104 (2011)]. We now show that this CIS error can be corrected approximately by performing a single Newton-Raphson step to reoptimize orbitals, thus establishing a new set of orbitals which better balances ground and excited state energies. The computational cost of this correction is exactly that of one coupled-perturbed Hartree-Fock calculation, which is effectively the cost of the CIS calculation itself. In other words, for twice the computational cost of a standard CIS calculation, or roughly the same cost as a linear-response time-dependent Hartree-Fock calculation, one can achieve a balanced, size-consistent description of CT versus non-CT energies, ideally with the accuracy of a much more expensive doubles CIS(D) calculation.
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Xinle
authors.initials: X
authors.lastname: Liu
authors.forename: Shervin
authors.initials: S
authors.lastname: Fatehi
authors.forename: Yihan
authors.initials: Y
authors.lastname: Shao
authors.forename: Brad S
authors.initials: BS
authors.lastname: Veldkamp
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Subotnik