A semiclassical route to the calculation of IR spectra

Semiclassical vibrational spectroscopy is based on the evolution of classical trajectories and is able to reproduce quantum effects with good accuracy at the cost of a reasonable computational effort. Nevertheless, semiclassical vibrational power spectra do not simulate all the features of the experimental IR spectra, since intensities in power spectra are not directly related to the IR absorptions. Therefore, we developed an innovative strategy with the aim of calculating IR spectra within the semiclassical framework. To do this, we started from the general definition of the absorption spectrum as the Fourier transform of the transition dipole moment autocorrelation function; we then added the Herman-Kluk approximation to the quantum propagator and made use of the time-averaging procedure to promote the convergence of the phase-space integrals. Finally, we tested the accuracy of this new method on some simple analytical systems and small molecules in the gas phase.