Interpretation of Coupled-Cluster Many-Electron Dynamics in Terms of Stationary States

TL;DR

This paper introduces a method to analyze laser-driven many-electron dynamics using stationary-state populations within coupled-cluster theory, validated through numerical tests on atoms and molecules, highlighting the EOM projector as most effective.

Contribution

It proposes and tests projectors based on linear response and equation-of-motion coupled-cluster theory for calculating stationary-state populations during laser interactions.

Findings

CCSD results closely match full configuration-interaction when relevant states are well approximated.

The EOM projector effectively avoids spurious oscillations in population calculations.

Accuracy decreases when double-excited states significantly influence the dynamics.

Abstract

We demonstrate theoretically and numerically that laser-driven many-electron dynamics, as described by bivariational time-dependent coupled-cluster theory, may be analyzed in terms of stationary-state populations. Projectors heuristically defined from linear response theory and equation-of-motion coupled-cluster theory are proposed for the calculation of stationary-state populations during interaction with laser pulses or other external forces, and conservation laws of the populations are discussed. Numerical tests of the proposed projectors, involving both linear and nonlinear optical processes for the He and Be atoms, and for the LiH, CH$^+$, and LiF molecules, show that the laser-driven evolution of the stationary-state populations at the coupled-cluster singles-and-doubles (CCSD) level is very close to that obtained by full configuration-interaction theory provided all stationary states actively participating in the dynamics are sufficiently well approximated. When double-excited states are important for the dynamics, the quality of the CCSD results deteriorate. Observing that populations computed from the linear-response projector may show spurious small-amplitude, high-frequency oscillations, the equation-of-motion projector emerges as the most promising approach to stationary-state populations.