Structure and stability of Mg+n and Mg++n clusters (n≤7)

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Abstract
We present a nonempirical model Hamiltonian of the valence bond type for singly and doubly charged clusters of atoms which have a s2(1S) ground state configuration. This model includes delocalization, instantaneous polarization energies, interaction between induced dipoles, and Coulombic repulsion of the holes in doubly charged clusters; up to four-body terms are included. The model is applied to magnesium clusters. Comparisons with ab initio (configuration interaction) calculations show overall agreement for singly charged clusters, whereas doubly charged clusters suffer from the lack of collective p-type contributions in model calculations. For singly charged magnesium clusters, ground state geometries are found to be neutral atoms surrounding either a charged atom or a charged dimer. Considerations of shape and energy show that clusters are easily obtained by aggregation processes and analysis of vertical ionization potentials shows that they can also be obtained by direct ionization of neutral species.