Investigations of diffuse intermolecular electronic systems
Muguet, Francis F.
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Diffuse intermolecular electronic systems, such as the hydrated electron or the immonia and water dimers, present both a theoretical and a practical computational challenge. The hydrated electron was discovered more than 25 years ago, yet there is still no consensus on an explanation of this phenomenon. A novel model is presented here whereby the hydrated electron consists in an itinerant dihydronium radical structure. Although electrostatically neutral, the itinerant radical is shown to behave as a negative charge carrier under the influence of an electric field. Within this perspective, the hydrated electron may be considered a quasiparticle. Contrary of the absence of agreement between many experiments and the old but still popular cavity model description, the energetics in the new model, are shown to be consistent with photophysical experimental data. In order to understand negatively charged water clusters, it is also proposed that a metastable bifurcated water dimer structure is able to bind an extra electron. Prior to our studies, no ab initio computations had been able to reproduce the experimental geometry of the ammonia dimer nor to predict a water dimer anion with Franck-Conden factors agreeing with those recently found in molecular beam experiments. In both cases the potential energy surface is determined by attractors corresponding to nonlinear and linear hydrogen bonded geometries, respectively. One attracter receives an unfair advantage in the computational procedure mainly because of the basis set superposition error (ESSE). There is still no agreement on a scheme for correcting the ESSE. A widely employed error estimation method is the counterpoise correction. A completely different new method is proposed using reerthonermalizatien of purified localized molecular orbitals. In terms of a ESSE corrected potential energy surface of the water dimer, a multi-attractor model of water is very briefly discussed. For further water molecular dynamics studies, we offer a new algorithm which we have developed specifically for a massively parallel computer.