# An Effective Semiclassical Approach to IR Spectroscopy

## TL;DR

This paper introduces a new semiclassical molecular dynamics method for calculating IR spectra that captures quantum effects, anharmonicities, and spectral shapes, enabling analysis of complex molecular systems without state-to-state calculations.

## Contribution

The novel approach improves IR spectral calculations by avoiding state-to-state computations and accurately including spectral shapes, quantum effects, and anharmonicities.

## Key findings

- Accurate IR spectra for water and glycine obtained
- Method captures spectral shapes and quantum effects
- Applicable to systems with dense vibrational states

## Abstract

We present a novel approach to calculate molecular IR spectra based on semiclassical molecular dynamics. The main advance from a previous semiclassical method [M. Micciarelli, R. Conte, J. Suarez, M. Ceotto J. Chem. Phys. 149, 064115 (2018)] consists in the possibility to avoid state-to-state calculations making applications to systems characterized by sizable densities of vibrational states feasible. Furthermore, this new method accounts not only for positions and intensities of the several absorption bands which make up the IR spectrum, but also for their shapes. We show that accurate semiclassical IR spectra including quantum effects and anharmonicities for both frequencies and intensities can be obtained starting from semiclassical power spectra. The approach is first tested against the water molecule, and then applied to the 10-atom glycine aminoacid.