Multi-reference approach to the computation of double core-hole spectra

Area of Science:

• Quantum Chemistry
• Molecular Spectroscopy
• Computational Physics

Background:

• Double Core-Hole (DCH) states are crucial for understanding electronic structure.
• Accurate theoretical descriptions require accounting for relaxation and correlation effects.
• Previous characterizations of K-2 shake-up spectra have been incomplete.

Purpose of the Study:

• To accurately assess Double Core-Hole (DCH) states in small molecules.
• To provide a detailed characterization of K-2 shake-up spectra.
• To validate computational methods against experimental and theoretical data.

Main Methods:

• Restricted active space self-consistent field (RASSCF) method.
• Multi-state restricted active space perturbation theory of second order (MS-RASPT2).
• State-interaction (SI) approximation with biorthonormalized molecular orbitals.

Main Results:

• Accurate binding energies and intensities for double-core-ionized states (K-2) were obtained.
• Excellent agreement with existing theoretical and experimental data was achieved.
• Complete characterization of K-2 shake-up spectra for H2O and C2H2n (n=1-3) was accomplished for the first time.

Conclusions:

• The employed computational methods provide accurate descriptions of DCH states.
• The study successfully characterized previously unassigned K-2 shake-up spectra.
• This work sets a benchmark for future studies on molecular electronic structure.