Inner-shell ionizations and satellites studied by the open-shell reference symmetry-adapted cluster/symmetry-adapted...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Accurate calculation of electronic structure, particularly for core-level properties, is essential in chemistry.
• Existing methods may face challenges in describing open-shell systems and achieving quantitative agreement with experimental ionization data.
• Symmetry-Adapted Cluster (SAC) and SAC-Configuration Interaction (CI) are established quantum chemical methods.

Purpose of the Study:

• To develop and validate open-shell reference (OR)-SAC and OR-SAC-CI methods for electronic structure calculations.
• To apply these novel methods to investigate the inner-shell ionizations of small molecules (CH4, NH3, H2O, HF).
• To assess the importance of electron correlation and orbital relaxation in achieving accurate ionization potentials and spectra.

Main Methods:

• Development of open-shell reference (OR) versions of the SAC and SAC-CI methods.
• Application of OR-SAC and OR-SAC-CI to calculate inner-shell ionization potentials and spectra.
• Comparison of calculated results with experimental data and with SAC-CI general-R calculations.

Main Results:

• The OR-SAC and OR-SAC-CI methods yield excellent agreement between calculated and experimental inner-shell ionization potentials and spectra.
• Both electron correlation and orbital relaxation effects are demonstrated to be critical for quantitative accuracy.
• The developed OR-SAC and OR-SAC-CI methods show comparable accuracy to SAC-CI general-R but with improved computational efficiency.

Conclusions:

• The newly developed OR-SAC and OR-SAC-CI methods are highly effective for studying inner-shell ionizations in open-shell systems.
• These methods provide a computationally efficient and accurate approach for theoretical spectroscopy.
• The findings underscore the necessity of incorporating electron correlation and orbital relaxation for precise predictions of molecular ionization phenomena.