Anharmonicity in Molecular Crystals: Generalized Perturbation Theory Meets Periodic Computations

Area of Science:

• Computational spectroscopy
• Solid-state physics
• Theoretical chemistry

Background:

• Simulating solid-state vibrational spectra is challenging due to anharmonicity, intermolecular interactions, and resonances.
• Accurate computational methods are needed for quantitative analysis of molecular solids.

Purpose of the Study:

• To introduce a generalized second-order vibrational perturbation theory (GVPT2) framework for molecular solids.
• To achieve accurate and efficient quantitative computational spectroscopy of solid-state systems.

Main Methods:

• Developed a generalized VPT2 (GVPT2) framework using a perturb-then-diagonalize approach.
• Excluded resonant terms in the initial perturbative treatment, handling them via a variational approach for stability and accuracy.
• Applied the method to simulate the infrared spectrum of solid carbon dioxide (dry ice).

Main Results:

• The GVPT2 approach accurately reproduced absolute band positions and splitting patterns for solid CO2.
• Results showed excellent agreement with experimental data, validating the method's accuracy.
• Demonstrated the method's ability to capture strong anharmonic effects and Fermi resonances.

Conclusions:

• The developed GVPT2 method provides a reliable and transferable approach for anharmonic vibrational analysis in molecular solids.
• This framework offers a significant advancement in computational spectroscopy for solid-state systems.
• The method's success with dry ice highlights its potential for diverse solid-state applications.