Cluster Perturbation Theory for Core Excited States and Core Ionization Potentials Using Core-Valence Separation.

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Theoretical Chemistry

Background:

• Accurate computational methods are crucial for analyzing modern X-ray spectroscopy experiments.
• Existing methods struggle with the computational cost of ab initio calculations for core excited states.

Purpose of the Study:

• To extend Cluster Perturbation theory (CPT) for calculating core excited states and ionization potentials.
• To introduce the core-valence separation approximation into CPT for excitation energy calculations.
• To assess the performance of the new methodology in S(D) orbital excitation spaces.

Main Methods:

• Developed a theoretical framework to incorporate core-valence separation into CPT for excitation energies.
• Applied the CPS(D) method to calculate core excitation energies for organic molecules.
• Compared CPS(D) results with coupled-cluster single and double (CCSD) reference calculations.

Main Results:

• The CPS(D) method accurately determines carbon, nitrogen, and oxygen K-edge excitation energies.
• Achieved errors below 2 eV compared to CCSD reference results.
• Demonstrated the applicability of CPS(D) to systems beyond the reach of conventional CCSD.

Conclusions:

• The developed CPS(D) excitation energy models provide accurate and efficient calculations for core spectroscopies.
• This advancement facilitates theoretical analysis of complex molecular systems in X-ray experiments.
• The CPS(D) method offers a computationally feasible approach for studying core-level phenomena.