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 NewtonRaphson 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.

Exciplexes and conical intersections lead to fluorescence quenching in π-stacked dimers of 2-aminopurine with natural purine nucleobases

“…73,74 For this reason we had to include many more states at the CIS level (30 states) to ensure that the CT states were included and could be corrected based on perturbation theory at the RI-CIS(D) level. We also noticed that some nπ* states were also overestimated at the CIS level, and RI-CIS(D) stabilized them considerably compared to other states.…”

Communication: An inexpensive, variational, almost black-box, almost size-consistent correction to configuration interaction singles for valence excited states

“…9 In fact, we demonstrated that a perturbative approach for reoptimizing orbitals (which we called OO-CIS) 9 yielded a large negative energy correction for CT states which was comparable to CIS(D). Nevertheless perturbation theory is never suitable near a crossing point.…”

Describing long-range charge-separation processes with subsystem density-functional theory

“…Furthermore, low-cost methods improving on CIS have recently been proposed by Subotnik and co-workers. [13][14][15] Another class of methods can be thought of as stabilizing charge-localized solutions by introducing (explicit or implicit) constraints in the electronic-structure calculation. Typically, self-consistent field (SCF) calculations will not converge to charge-separated solutions, as these are energetically unfavorable and correspond to "excited" configurations of the system unless stabilized by external effects.…”