EOM-CC methods with approximate triple excitations applied to core excitation and ionisation energies

Abstract

A number of iterative and perturbative approximations to the full equation-of-motion coupled cluster method with single, double, and triple excitations (EOM-CCSDT) are evaluated in the context of calculating the K-edge core-excitation and core-ionisation energies of several small molecules. Several of these methods are found to accurately reproduce the full EOM-CCSDT energies well, in particular the EOM-CCSD* method which, when using the core-valence separation (CVS) ansatz, scales rigorously with the sixth power of molecular size. These results highlight that very accurate core excitation and ionisation energies for molecules with first-row atoms can be computed at a cost not much larger than that for EOM-CCSD.

GRAPHICAL ABSTRACT

Keywords:

• Coupled cluster
• NEXAFS
• XPS
• excited states

Acknowledgements

DAM would like to acknowledge a generous start-up package from SMU.

Disclosure statement

No potential conflict of interest was reported by the author.

ORCID

Devin A. Matthews http://orcid.org/0000-0003-2795-5483

Notes

1 Of course, this analysis assumes that only a constant number (i.e. not growing with molecular size) of core orbitals are included in the CVS treatment.