Absorption spectrum calculations using mixed quantum-Gaussian wave packet dynamics

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Abstract
We calculate the absorption spectrum of a cluster using a computational method in which classical-like degrees of freedom for the nuclei are described by Gaussian wave packets while the valence electrons are treated quantum mechanically. We examine the spectral features in comparison to an even simpler mixed quantum-classical model in which the nuclear motion is treated by purely classical mechanics. Anomalous features (such as negative absorption) in the absorption spectrum which can arise from mixed quantum-classical methods are examined and the Gaussian wave packet nuclear dynamics is found to substantially reduce these anomalous features. This method is applied to a two-coordinate model problem in which exact numerical results can be obtained and we find that the method works fairly well. We also apply the method to the valence electronic absorption spectrum for a KXe6 cluster. The method does not suffer from the dramatic failure seen when Xe motion is treated classically. The method is used to calculate the vibrational width and the vibrational structure of the electronic absorption spectrum.