Multireference Equation-of-Motion-Driven Similarity Renormalization Group for X-ray Photoelectron Spectra.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Simulating X-ray photoelectron spectra (XPS) of strongly correlated molecular systems is computationally challenging.
• Existing methods often struggle with accuracy for complex electronic structures.
• Accurate prediction of core-ionization energies is crucial for understanding molecular electronic properties.

Purpose of the Study:

• To formulate and implement a robust and efficient computational method for XPS simulation.
• To accurately calculate core-ionization energies for strongly correlated molecules.
• To investigate the applicability of the new method to molecular dissociation and vibrational structure.

Main Methods:

• Development of the core-valence separated multireference equation-of-motion-driven similarity renormalization group (CVS-IP-EOM-DSRG) method.
• Implementation of three theoretical variants: DSRG-MRPT2, DSRG-MRPT3, and MR-LDSRG(2).
• Benchmarking against established single-reference and multireference methods using molecular test sets.

Main Results:

• The CVS-IP-EOM-DSRG method demonstrates numerical robustness and computational efficiency with O(N^4) scaling.
• All three variants accurately predict vertical core-ionization energies.
• DSRG-MRPT3 and MR-LDSRG(2) variants successfully capture molecular dissociation behavior and reproduce experimental vibrational structures.

Conclusions:

• The developed CVS-IP-EOM-DSRG method is a powerful tool for simulating XPS of strongly correlated systems.
• Higher-level approximations (DSRG-MRPT3, MR-LDSRG(2)) are essential for accurately describing dissociation and vibrational spectra.
• This method provides accurate and efficient simulation of core-ionization energies and molecular dynamics.