Fermi resonance in CO₂: Mode assignment and quantum nuclear effects from first principles molecular dynamics

Area of Science:

• Computational Chemistry
• Spectroscopy
• Condensed Matter Physics

Background:

• Vibrational spectroscopy is key for studying atomic structures and environmental effects.
• Assigning spectral features, especially double peaks like Fermi dyad resonances, is challenging in simulations and experiments.
• Fermi dyad resonances arise from the interaction between a fundamental excitation and an overtone.

Purpose of the Study:

• To present an efficient computational approach for unambiguous characterization of Fermi resonances in condensed phase systems using DFT.
• To enable accurate assignment of spectral features and identification of resonating modes.
• To incorporate thermal quantum nuclear effects into simulations for improved accuracy.

Main Methods:

• Developed a new method within Density Functional Theory (DFT) for simulating condensed phase systems.
• Post-processed DFT data from classical nuclear dynamics simulations.
• Combined classical dynamics with a perturbative quantum treatment at finite temperatures to include thermal quantum nuclear effects.

Main Results:

• Successfully characterized Fermi resonances and identified the two resonating modes.
• Demonstrated that thermal quantum nuclear effects significantly improve the accuracy of spectral feature prediction, particularly frequency splitting.
• Validated the approach against experimental data for supercritical CO2, showing substantial effects even at ambient conditions.

Conclusions:

• The proposed DFT-based scheme accurately accounts for Fermi resonances and thermal quantum nuclear effects.
• This method provides a computationally convenient and accurate way to interpret vibrational spectra.
• The findings are crucial for understanding molecular behavior in various environments, including supercritical fluids.