Non-iterative triples excitations in equation-of-motion coupled-cluster theory for electron attachment with applications to bound and temporary anions

TL;DR

This paper introduces a non-iterative triples excitation method within equation-of-motion coupled-cluster theory for electron attachment, improving the accuracy of resonance positions and widths in bound and temporary anions.

Contribution

The study presents the EOM-CCSD(T)(a)* method, which efficiently incorporates triples excitations, enhancing the accuracy of electron attachment calculations over previous approaches.

Findings

EOM-CCSD(T)(a)* reliably reproduces vertical attachment energies.

Shape resonances are well described by EOM-EA-CCSD.

Residual electron correlation significantly impacts resonance positions and widths.

Abstract

The impact of residual electron correlation beyond the equation-of-motion coupled-cluster singles and doubles approximation (EOM-CCSD) on positions and widths of electronic resonances is investigated. To establish a method that accomplishes this task in an economical manner, several approaches proposed for the approximate treatment of triples excitations are reviewed with respect to their performance in the electron attachment (EA) variant of EOM-CC theory. The recently introduced EOM-CCSD(T)(a)* method, which includes non-iterative corrections to the reference and the target states, reliably reproduces vertical attachment energies from EOM-EA-CC calculations with singles, doubles, and full triples excitations in contrast to schemes in which non-iterative corrections are applied only to the target states. Applications of EOM-EA-CCSD(T)(a)* augmented by a complex absorbing potential (CAP) to several temporary anions illustrate that shape resonances are well described by EOM-EA-CCSD, but that residual electron correlation often makes a non-negligible impact on their positions and widths. The positions of Feshbach resonances, on the other hand, are significantly improved when going from CAP-EOM-EA-CCSD to CAP-EOM-EA-CCSD(T)(a)*, but the correct energetic order of the relevant electronic states is still not achieved.