Transition-potential coupled cluster II: optimisation of the core orbital occupation number

Abstract

The issue of orbital relaxation in computational core-hole spectroscopy, specifically x-ray absorption, has been a major problem for methods such as equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). The transition-potential coupled cluster (TP-CC) method is utilised to address this problem by including an explicit treatment of orbital relaxation via the use of reference orbitals with a fractional core occupation number. The value of the fractional occupation parameter λ was optimised for both TP-CCSD and XTP-CCSD methods in an element-specific manner due to the differences in atomic charge and energy scale. Additionally, TP-CCSD calculations using the optimised parameters were performed for the K-edge absorption spectra of gas-phase adenine and thymine. TP-CCSD reproduces the valence region well and requires smaller overall energy shifts in comparison to EOM-CCSD, while also improving on the relative position and intensities of several absorption peaks.

GRAPHICAL ABSTRACT

Keywords:

• Coupled cluster
• NEXAFS
• XPS
• excited states

Acknowledgments

MS is supported by an SMU Center for Research Computing Graduate Fellowship. All calculations were performed on the ManeFrame II computing system at SMU.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported in part by the US National Science Foundation under grant OAC-2003931.