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.
@article{arxiv.2011.03595,
title = {Transition-Potential Coupled Cluster},
author = {Megan E. Simons and Devin A. Matthews},
journal= {arXiv preprint arXiv:2011.03595},
year = {2025}
}