Reference Energies for Non-Relativistic Core Ionization Potentials

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Electronic Structure Theory

Background:

• Core electrons provide site-specific information crucial for X-ray photoelectron spectroscopy.
• Predicting core ionization potentials (IPs) theoretically is complex due to orbital relaxation, electron correlation, and relativistic effects.
• Previous assessments relied on experimental data, entangling various error sources and hindering evaluation of theoretical methods.

Purpose of the Study:

• To establish a consistent, theory-based benchmark for core IPs.
• To provide chemically accurate reference data for developing and validating computational methods.
• To enable systematic theory-versus-theory comparisons to isolate correlation and relaxation effects.

Main Methods:

• Computed 84 nonrelativistic core IPs (73 second-row, 11 third-row) using the core-valence separation approximation.
• Employed the full configuration interaction (FCI) level of theory.
• Utilized large correlation-consistent basis sets (aug-cc-pCVXZ) with tight-core and diffuse functions.

Main Results:

• Generated theoretical best estimates for core IPs within a fixed finite basis set.
• Created a dataset enabling disentanglement of correlation and relaxation effects from other physical contributions.
• Assessed the performance of approximate methods like equation-of-motion coupled-cluster and G0W0.

Conclusions:

• The FCI benchmark provides a reliable reference for evaluating theoretical methods for core IPs.
• This work facilitates a clearer understanding of the accuracy of various computational approaches.
• The established benchmark aids in the development of more precise theoretical tools for electronic structure calculations.