Extended Configuration-Interaction Singles Method with Core/Valence Separation (XCIS-CVS): Core-Level Spectra of...

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Electronic Structure Theory

Background:

• Core-to-valence transitions probe valence virtual orbitals, crucial for open-shell systems with degenerate frontier orbitals.
• Standard methods like time-dependent density functional theory struggle with spin contamination in open-shell excited state calculations.
• Restricted open-shell Hartree-Fock based configuration interaction singles (ROHF-CIS) often yields inaccurate results due to spin contamination.

Purpose of the Study:

• To implement and apply the Extended Configuration-Interaction Singles (XCIS) method with the core/valence separation (CVS) approximation.
• To accurately model spin-pure excited states and core-to-valence transitions in open-shell molecules.
• To assess the performance of XCIS-CVS for simulating X-ray transitions in 3d transition metal complexes.

Main Methods:

• Implementation of the Extended Configuration-Interaction Singles (XCIS) wave function ansatz.
• Incorporation of the core/valence separation (CVS) approximation to focus on relevant orbitals.
• Application to simulate K-edge and pre-edge X-ray transitions in various open-shell systems.

Main Results:

• The XCIS method effectively eliminates spin contamination present in ROHF-CIS calculations.
• XCIS-CVS provides improved accuracy for excited states compared to standard CIS methods.
• Simulated K-edge and pre-edge orbital splittings for 3d transition metal complexes show semiquantitative agreement with experimental data.

Conclusions:

• XCIS-CVS is a robust and accurate method for studying excited states and spectroscopic transitions in open-shell systems.
• The method successfully addresses the limitations of single-excitation theories for challenging open-shell molecules.
• XCIS-CVS offers a reliable computational tool for interpreting X-ray spectroscopy of transition metal complexes.