Geometric phase in coupled cluster theory

David M G Williams; Eirik F Kjønstad; Todd J Martínez

doi:10.1063/5.0151856

Geometric phase in coupled cluster theory

J Chem Phys. 2023 Jun 7;158(21):214122. doi: 10.1063/5.0151856.

Authors

David M G Williams¹, Eirik F Kjønstad¹, Todd J Martínez¹

Affiliation

¹ Department of Chemistry and the PULSE Institute, Stanford University, Stanford, California 94305, USA and SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.

PMID: 37283267
DOI: 10.1063/5.0151856

Abstract

It has been well-established that the topography around conical intersections between excited electronic states is incorrectly described by coupled cluster and many other single reference theories (the intersections are "defective"). Despite this, we show both analytically and numerically that the geometric phase effect (GPE) is correctly reproduced upon traversing a path around a defective excited-state conical intersection (CI) in coupled cluster theory. The theoretical analysis is carried out by using a non-Hermitian generalization of the linear vibronic coupling approach. Interestingly, the approach qualitatively explains the characteristic (incorrect) shape of the defective CIs and CI seams. Moreover, the validity of the approach and the presence of the GPE indicate that defective CIs are local (and not global) artifacts. This implies that a sufficiently accurate coupled cluster method could predict nuclear dynamics, including geometric phase effects, as long as the nuclear wavepacket never gets too close to the conical intersections.