Double Configuration Interaction Singles: Scalable and size-intensive approach for orbital relaxation in excited states and bond-dissociation

TL;DR

This paper introduces a scalable, size-intensive perturbative approach to improve orbital relaxation in CIS, enhancing accuracy for excited states and bond dissociation at mean-field computational cost.

Contribution

The authors develop a novel perturbative scheme for orbital relaxation in CIS, incorporating de-excitation effects, which improves accuracy without increasing computational complexity.

Findings

Reduces overestimation of charge-transfer excitation energies.

Effectively describes single bond dissociation.

Maintains size-intensive property of CIS.

Abstract

We present a novel theoretical scheme for orbital relaxation in configuration interaction singles (CIS) based on a perturbative treatment of its electronic Hessian, whose analytical derivation is also established in this work. The proposed method, which can be interpreted as a "CIS-then-CIS" scheme, variationally accounts for orbital relaxation in excited states, thus significantly reducing the overestimation of charge-transfer excitation energies commonly associated with standard CIS. Additionally, by incorporating de-excitation effects from CIS, we demonstrate that our approach effectively describes single bond dissociation. Notably, all these improvements are achieved at a mean-field cost, with the pre-factor further reduced with the efficient algorithm introduced here, while preserving the size-intensive property of CIS.