Resolving the notorious case of conical intersections for coupled cluster dynamics

TL;DR

This paper introduces a novel coupled cluster theory formulation that accurately describes conical intersections between excited electronic states, enabling better modeling of nonadiabatic photochemical processes.

Contribution

It presents the first coupled cluster approach capable of correctly modeling conical intersections between same-symmetry excited states.

Findings

Enables accurate simulation of nonadiabatic dynamics involving conical intersections

Demonstrates applicability of coupled cluster theory to excited state processes

Addresses a long-standing challenge in quantum chemical modeling

Abstract

The motion of electrons and nuclei in photochemical events often involve conical intersections, degeneracies between electronic states. They serve as funnels for nuclear relaxation - on the femtosecond scale - in processes where the electrons and nuclei couple nonadiabatically. Accurate ab initio quantum chemical models are essential for interpreting experimental measurements of such phenomena. In this paper we resolve a long-standing problem in coupled cluster theory, presenting the first formulation of the theory that correctly describes conical intersections between excited electronic states of the same symmetry. This new development demonstrates that the highly accurate coupled cluster theory can be applied to describe dynamics on excited electronic states involving conical intersections.