An efficient pair natural orbital based configuration interaction scheme for the calculation of open-shell ionization...

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Electronic Structure Theory

Background:

• High spin open shell systems present challenges for accurate electronic structure calculations.
• Evaluating ionization potentials is crucial for understanding photoionization processes.
• Previous methods for X-ray absorption spectra utilized averaged pair natural orbital techniques.

Purpose of the Study:

• To develop a spin-adapted configuration interaction scheme for ionization potential calculations in high spin open shell systems.
• To enhance computational efficiency through virtual space reduction and approximate integral evaluation.
• To enable qualitative characterization of X-ray photoionization spectra and vibrational transitions.

Main Methods:

• A spin-adapted configuration interaction (CI) scheme was developed.
• Ionization operators were constructed for alpha and beta electron removal.
• Averaged pair natural orbital (APNO) machinery was employed for virtual space reduction.
• Resolution of identity and seminumerical techniques were used for integral evaluation.
• Path integral techniques were incorporated for vibrational effects.

Main Results:

• The proposed method allows for the evaluation of ionization potentials in high spin open shell systems.
• Computational efficiency was improved using APNOs and approximate integral schemes.
• The method provides qualitative insights into X-ray photoionization spectra features.
• Vibrational transitions corresponding to main peaks can be computed.

Conclusions:

• The developed spin-adapted CI scheme offers a computationally efficient approach for studying ionization potentials in high spin systems.
• While not yielding highly accurate absolute energies, the method is valuable for qualitative spectral analysis.
• The inclusion of vibrational effects expands its applicability to understanding spectral peak transitions.