Delta-coupled-cluster methods for accurate calculations of core ionization energies

We report scalar-relativistic Delta-coupled-cluster (ΔCC) methods with core-valence separation (CVS) approximation^[1] to study single core ionization energies ^[2] and hetero-site double core ionization energies ^[3] for second-row elements. ΔCCSD(T) is shown to be as accurate as CVS-equation-of-motion coupled-cluster with higher quadruple excitations^[4,5] in benchmark calculations of single core ionization energies ^[2]. ΔCC methods are also capable of providing accurate double core ionization energies with remaining errors estimated to be below 0.3 eV. They appear to be promising tools to facilitate experimental studies of two-site double core-ionized states that are involved in X-ray pump/X-ray probe studies of electronic and molecular dynamics following inner shell ionization or excitation.

Reference
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[2] X. Zheng and L. Cheng, J. Chem. Theory Comput. 15, 4945-4955 (2019).
[3] X. Zheng, J. Liu, G. Doumy, L. Young, and L. Cheng, J. Phys. Chem. A 124, 4413-4426 (2020).
[4] S. Coriani, and H. Koch, J. Chem. Phys. 143, 181103 (2015).
[5] J. Liu , D. Matthews, S. Coriani and L. Cheng, J. Chem. Theory Comput. 15, 1642–1651 (2019).