Combining Static and Dynamical Approaches for Infrared Spectra Calculations of Gas Phase Molecules and Clusters.

Four models for the calculation of the IR spectrum of gas phase molecules and clusters from molecular dynamics simulations are presented with the aim to reduce the computational cost of the usual Fourier transform (FT) of the time correlation function of the dipole moment. These models are based on the VDOS, FT of time correlation function of velocities, and atomic polar tensors (APT). The models differ from each other by the number of APTs inserted into the velocities correlation function. Excellent accuracy is achieved by the model adopting a weighted linear combination of a few selected APTs adapted for the rotation of the molecule (model D). The achieved accuracy relates to band positions, band shapes, and band intensities. Depending on the degree of actual dynamics of the molecule, rotational motion, conformational isomerization, and large amplitude motions that can be seen during the finite temperature trajectory, one could also apply one of the other models (models A, B, or C), but with caution. Model D is therefore found simple and accurate, with appealing computational cost and should be systematically applied. Its generalization to condensed phase systems should be straightforward.