Time-dependent equation-of-motion coupled-cluster simulations with a defective Hamiltonian

Stephen H Yuwono; Brandon C Cooper; Tianyuan Zhang; Xiaosong Li; A Eugene DePrince

doi:10.1063/5.0157852

Time-dependent equation-of-motion coupled-cluster simulations with a defective Hamiltonian

J Chem Phys. 2023 Jul 28;159(4):044113. doi: 10.1063/5.0157852.

Authors

Stephen H Yuwono¹, Brandon C Cooper¹, Tianyuan Zhang², Xiaosong Li², A Eugene DePrince¹

Affiliations

¹ Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA.
² Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.

PMID: 37497820
DOI: 10.1063/5.0157852

Abstract

Simulations of laser-induced electron dynamics in a molecular system are performed using time-dependent (TD) equation-of-motion (EOM) coupled-cluster (CC) theory. The target system has been chosen to highlight potential shortcomings of truncated TD-EOM-CC methods [represented in this work by TD-EOM-CC with single and double excitations (TD-EOM-CCSD)], where unphysical spectroscopic features can emerge. Specifically, we explore driven resonant electronic excitations in magnesium fluoride in the proximity of an avoided crossing. Near the avoided crossing, the CCSD similarity-transformed Hamiltonian is defective, meaning that it has complex eigenvalues, and oscillator strengths may take on negative values. When an external field is applied to drive transitions to states exhibiting these traits, unphysical dynamics are observed. For example, the stationary states that make up the time-dependent state acquire populations that can be negative, exceed one, or even complex-valued.