Modern computers routinely allow efficient calculation of the geometries and electronic structures of neutral molecules using quantum-mechanical principles. However, charged species present a considerably greater challenge, and excess electrons are often treated by means of classical approximations. Herbert and Head-Gordon describe a method whereby the spatial distribution and detachment energy of an excess electron bound to a tetrameric water cluster can be computed quantum-mechanically. These hydrated water clusters have been the subject of extensive recent experimental study in light of the fundamental questions they raise about bonding motifs, as well as their role as models for bulk hydrated electrons of interest in biological electron transfer and photodamage.
To render the method computationally tractable, the authors propagate the cluster atoms along a classical trajectory while applying ab initio Møller-Plesset perturbation theory at each step to solve the electronic structure. The simulation results agree well with recent experimental measurements of vibrational and photoelectron spectra, and furthermore allow estimation of the cluster temperatures based on observed spectral widths. The authors note in closing that further advances in computing power should extend the applicability of the method to larger molecular clusters. -- JSY
Proc. Natl. Acad. Sci. U.S.A. 103, 10.1073/pnas.0603679103 (2006).
