N-representability violations in truncated equation-of-motion coupled-cluster methods

Stephen H Yuwono; A Eugene DePrince

doi:10.1063/5.0157854

N-representability violations in truncated equation-of-motion coupled-cluster methods

J Chem Phys. 2023 Aug 7;159(5):054113. doi: 10.1063/5.0157854.

Authors

Stephen H Yuwono¹, A Eugene DePrince¹

Affiliation

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

PMID: 37548301
DOI: 10.1063/5.0157854

Abstract

One-electron reduced density matrices (1RDMs) from equation-of-motion (EOM) coupled-cluster with single and double excitations (CCSD) calculations are analyzed to assess their N-representability (i.e., whether they are derivable from a physical N-electron state). We identify EOM-CCSD stationary states whose 1RDMs violate either ensemble-state N-representability conditions or pure-state conditions known as generalized Pauli constraints. As such, these 1RDMs do not correspond to any physical N-electron state. Unphysical states are also encountered in the course of time-dependent EOM-CC simulations; when an external field drives transitions between a pair of stationary states with pure-state N-representable 1RDMs, the 1RDM of the time-dependent state can violate ensemble-state conditions. These observations point to potential challenges in interpreting the results of time-dependent EOM-CCSD simulations.