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

Yuwono, Stephen H.; DePrince, III, A. Eugene

doi:10.1063/5.0157854

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

Yuwono, Stephen H.·DePrince, III, A. Eugene

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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. Furthermore, these observations point to potential challenges in interpreting the results of time-dependent EOM-CCSD simulations.

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY Coupled-cluster methods Density-matrix Excitation energies Occupation number Oscillator strengths Quantum chemistry Transition moment

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Yuwono, Stephen H., DePrince, III, A. Eugene. 2023-08-07. N -representability violations in truncated equation-of-motion coupled-cluster methods. https://doi.org/10.1063/5.0157854

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