Accurate core excitation and ionization energies from a state-specific coupled-cluster singles and doubles approach

TL;DR

This paper introduces a state-specific coupled-cluster approach with orbital optimization for accurately calculating core excitation and ionization energies, outperforming traditional methods and reducing errors significantly.

Contribution

It develops and tests new Delta-CCSD schemes that improve accuracy of core excitation and ionization energy predictions without response theory or EOM formalisms.

Findings

Delta-CCSD schemes achieve errors comparable to experimental data.

Proposed methods outperform FC-CVS-EOM-CCSD with over twofold error reduction.

Methods are effective for small organic molecules without response theory.

Abstract

We investigate the use of orbital-optimized references in conjunction with single-reference coupled-cluster theory with single and double substitutions (CCSD) for the study of core excitations and ionizations of 18 small organic molecules, without any use of response theory or equation-of-motion formalisms. Three schemes are employed to successfully address the convergence difficulties associated with the coupled-cluster equations, and the spin contamination resulting from the use of a spin symmetry-broken reference, in the case of excitations. In order to gauge the inherent potential of the methods studied, an effort is made to provide reasonable basis set limit estimates for the transition energies. Overall, we find that the two best-performing schemes studied here for Delta-CCSD are capable of predicting excitation and ionization energies with errors comparable to experimental accuracies. The proposed Delta-CCSD schemes seem to fare better than the widely used equation-of-motion CCSD (EOM-CCSD) with core-valence separation protocol, with statistical errors being reduced by more than a factor of two when compared to FC-CVS-EOM-CCSD.