Generalized coupled cluster theory for ground and excited state intersections

TL;DR

This paper introduces a generalized coupled cluster framework that accurately describes ground and excited state conical intersections, overcoming previous limitations and enabling better modeling of nonadiabatic processes.

Contribution

The authors develop a new coupled cluster approach that correctly handles ground state intersections and geometric phase effects, expanding the applicability of coupled cluster methods.

Findings

Successfully describes ground state conical intersections

Resolves bifurcation issues in ground state equations

Demonstrates applicability to electronic-structure calculations

Abstract

Coupled cluster theory in the standard formulation is unable to correctly describe conical intersections among states of the same symmetry. This limitation has restricted the practical application of an otherwise highly accurate electronic structure model, particularly in nonadiabatic dynamics. Recently, the intersection problem among the excited states was fully characterized and resolved. However, intersections with the ground state remain an open challenge, and addressing this problem is our objective here. We present a generalized coupled cluster framework that correctly accounts for the geometric phase effect and avoids bifurcations of the solutions to the ground state equations. Several applications are presented that demonstrate the correct description of ground state conical intersections. We also propose how the framework can be used for other electronic-structure methods.