Singly and doubly charged clusters of (S2) metals. Ab initio and model calculations on Mg+n and Mgn++n(n ≤ 5)

Publisher Website
Google Scholar
Abstract
Accurate ab initio CI calculations on Mg+2 and Mg ++2 have been performed. The second appears to have a local minimum. Through a diabatization procedure, it was possible to define a simple model Hamiltonian spanned by products of atomic s2, s1 and s0 configurations. The model Hamiltonian includes instantaneous polarization energies (and three-body effects) and Coulombic repulsion of the holes in doubly charged clusters, and may be considered as treating one or two active holes in the polarizable s band. The model predicts linear Mg+3 and Mg++3 structures and nearly degenerate square and centred equilateral triangular structures for Mg+4 and Mg++4; Mg+5 and Mg++ 5 both present nearly degenerate minima of centred tetrahedron and centred square structures. The existence of such structures has been confirmed by UHF ab initio (gradient + force) calculations; it is shown however that single-determinantal approaches overestimate the bond lengths and unduly favour the centred structures; the Mg++5 minimum is 1.71 eV above the (Mg++ 3 + Mg+2) dissociation limit and 1.41 eV under the (3 Mg + 2 Mg+) dissociation limit. The vertical Ionisation Potentials (IP) have been calculated and it is shown that Mg+ 4, Mg+5 and Mg++5 may evaporate a neutral atom