Reference Energies for Non-Relativistic Core Ionization Potentials

TL;DR

This paper establishes a high-accuracy, theory-based benchmark dataset of 84 non-relativistic core ionization potentials for second- and third-row elements, aiding method validation and development.

Contribution

It provides the first comprehensive, consistent set of reference core IPs computed at the full configuration interaction level with large basis sets, enabling clear theory-versus-theory comparisons.

Findings

Benchmark dataset of 84 non-relativistic core IPs established.

Assessment of approximate methods like EOM-CC and G0W0 against high-level references.

Disentanglement of correlation and relaxation effects in core ionization energies.

Abstract

Deep-lying core electrons carry highly localized, site-specific information that forms the basis of X-ray photoelectron spectroscopy. Accurately predicting their associated core ionization potentials (IPs) is a demanding theoretical task, requiring a balanced treatment of strong orbital relaxation, electron correlation, and relativistic effects. Over the years, a variety of approaches have been developed, ranging from state-specific wave function methods to linear-response formalisms and Green's function techniques. However, their assessment has often relied on comparisons with experiment, where multiple sources of error (basis set incompleteness, relativistic corrections, and vibrational effects) are entangled, making it difficult to isolate the performance of correlation treatments. In the present work, we establish a consistent, theory-based benchmark for core IPs by computing 84 non-relativistic values (73 second-row and 11 third-row IPs) at the full configuration interaction level within the core-valence separation approximation, using large correlation-consistent basis sets augmented with tight-core and diffuse functions (aug-cc-pCVXZ). These results define theoretical best estimates within a fixed finite basis set, providing a chemically accurate reference for method development and validation. Importantly, our dataset allows for systematic, theory-versus-theory comparisons that disentangle correlation and relaxation effects from other physical contributions. On this basis, we assess the performance of widely used approximate methods, including equation-of-motion coupled-cluster approaches up to the inclusion of quadruple excitations, the one-shot $G_0W_0$ scheme, as well as state-specific methods.