Hetero-site Double Core Ionization Energies with Sub-electronvolt Accuracy from Delta-Coupled-Cluster Calculations

Area of Science:

• Quantum Chemistry
• Atomic and Molecular Physics
• Computational Chemistry

Background:

• Double core ionization involves removing two core electrons, significantly altering molecular electronic structure.
• Understanding these processes is crucial for interpreting X-ray spectroscopy and dynamics studies.
• Previous methods faced challenges in accurately predicting these energies, especially for hetero-site ionization.

Purpose of the Study:

• To perform benchmark calculations of double core ionization energies for small molecules containing second-row elements.
• To investigate the high-spin triplet components of two-site double core-ionized states.
• To assess the accuracy and reliability of delta-coupled-cluster (ΔCC) methods for these systems.

Main Methods:

• Scalar-relativistic delta-coupled-cluster (ΔCC) calculations were employed.
• Focus was on high-spin triplet states, which are single-reference in character.
• Analysis included contributions from electron correlation, basis-set effects, and core-valence separation approximations.

Main Results:

• ΔCC calculations demonstrated high accuracy for double core ionization energies, with errors below 0.3 eV.
• Systematic convergence of results with respect to computational effects was observed.
• The study validated the suitability of ΔCC for accurate predictions.

Conclusions:

• Scalar-relativistic ΔCC methods are recommended for accurate determination of double core ionization energies.
• These computational results can guide and facilitate experimental investigations.
• The findings are particularly relevant for X-ray pump/X-ray probe studies of electronic dynamics.