An extended semiclassical initial value representation approach to IR spectroscopy.

Area of Science:

• Computational Chemistry
• Spectroscopy
• Molecular Modeling

Background:

• Previous time-averaged infrared (IR) spectroscopy methods provided accurate semiclassical estimates for small molecules.
• These methods required thousands of trajectory calculations, limiting their application to small, few-atom systems.
• Accurate calculation of IR absorption intensities and transition frequencies is crucial for molecular characterization.

Purpose of the Study:

• To extend the applicability of time-averaged IR spectroscopy to larger molecules.
• To develop a more computationally efficient approach for calculating IR spectra.
• To validate the accuracy of the extended method on both small and larger molecular systems.

Main Methods:

• Transitioned from Monte Carlo integration of thousands of trajectories to tailored single-pair trajectory calculations.
• Introduced a partially time-independent approximation for the real part of the coherent state overlap.
• Tested the method's accuracy on water, formaldehyde, and methane before applying it to ethanol and glycine.

Main Results:

• The extended method successfully calculated IR spectra for ethanol and glycine.
• Vibrational intensities and frequencies obtained were found to be fairly accurate.
• The new approach significantly reduces computational cost compared to previous methods.

Conclusions:

• The developed method effectively extends accurate semiclassical IR spectroscopy to larger molecules.
• This approach offers a more feasible route for analyzing the vibrational properties of complex molecular systems.
• The method can be straightforwardly applied to an even wider range of molecular systems in future studies.