Wavefunction-based quantum-chemicalab initiocalculations for core electron binding energies of small open shell...

Area of Science:

• Quantum Chemistry
• Atomic and Molecular Physics
• Computational Chemistry

Background:

• Core electron binding energies (CEBEs) are fundamental properties of atoms and molecules.
• Accurate theoretical calculations are essential for interpreting experimental data and predicting molecular behavior.
• Open-shell molecules present unique challenges due to their electronic structure and spin states.

Purpose of the Study:

• To calculate CEBEs for small open-shell molecules containing first-row atoms using *ab initio* methods.
• To assess the accuracy of different theoretical approaches, including Koopmans' theorem, ΔSCF, and multi-reference methods.
• To provide reliable theoretical CEBE data for molecules where experimental gas-phase data is limited.

Main Methods:

• Wavefunction-based quantum-chemical *ab initio* calculations.
• Three-step approach: Koopmans' theorem, ΔSCF, and inclusion of dynamic correlation effects.
• Multi-reference complete active space self-consistent field (CAS-SCF) and coupled electron pair approximation for complex electronic states.

Main Results:

• Calculated CEBEs exhibit an accuracy of 0.1–0.4 eV.
• Results show good agreement with experimental data for NO and O₂.
• Demonstrated the necessity of multi-reference methods for accurately describing ionic states in open-shell systems.

Conclusions:

• The employed *ab initio* methods provide accurate CEBEs for open-shell molecules.
• Theoretical calculations are valuable for complementing scarce experimental data in this field.
• The study advances the understanding of electronic structure in open-shell molecular systems.