Coupled cluster (CC) is a numerical method used for describing many-body systems and is classified as one of the post-Hartree-Fock ab initio methods. The purpose of this method is to describe electron correlation in a wave function. The coupled cluster method is preferred due to its size-extensivity and implicit inclusion of higher-order excitation effects. The accuracy of the CC wave function can be controlled by truncating the cluster operator, T, at increasing levels of excitation; thus we use the coupled cluster singles, doubles, and triples method (CCSDT) as a benchmark for a range of x-ray absorption spectroscopy (XAS) calculations using only singles and doubles or with perturbative triple excitations. Our XAS benchmark involves transition of an electron from the 1s (K) orbital to a number of excited states in 14 small molecules. The problem of orbital relaxation in computational core-hole spectroscopies, including x-ray absorption and x-ray photoionization, has long plagued linear response approaches, including equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). Instead of addressing this problem by including additional electron correlation, we propose an explicit treatment of orbital relaxation via the use of "transition potential" reference orbitals, leading to a transition-potential coupled cluster (TP-CC) family of methods. One member of this family in particular, TP-CCSD(1/2), is found to essentially eliminate the orbital relaxation error and achieve the same level of accuracy for core-hole spectra as is typically expected of EOM-CCSD in the valence region. These results show that very accurate x-ray absorption spectra for molecules with first-row atoms can be computed at a cost essentially the same as that for EOM-CCSD. Starting from TP-CCSD(1/2), we have further tuned the core electron fraction in order to improve spectroscopic agreement for all first-row atoms. We have also expanded our benchmark calculations to include larger molecules, experimental spectra, and more computational methods such as ADC and STEOM.

Transition-potential coupled cluster

ACS Spring 2022

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