HM⁺-RG complexes (M = group 2 metal; RG = rare gas): Physical vs. chemical interactions

Previous work on the HM(+)-He complexes (M = Be-Ra) has been extended to the cases of the heavier rare gas atoms, HM(+)-RG (RG = Ne-Rn). Optimized geometries and harmonic vibrational frequencies have been calculated using MP2 theory and quadruple-ζ quality basis sets. Dissociation energies for the loss of the rare gas atom have been calculated at these optimized geometries using coupled cluster with single and double excitations and perturbative triples, CCSD(T)theory, extrapolating interaction energies to the basis set limit. Comparisons are made between the present data and the previously obtained helium results, as well as to those of the bare HM(+) molecules; furthermore, comparisons are made to the related M(+)-RG and M(2+)-RG complexes. Partial atomic charge analyses have also been undertaken, and these used to test a simple charge-induced dipole model. Molecular orbital diagrams are presented together with contour plots of the natural orbitals from the quadratic configuration with single and double excitations (QCISD) density. The conclusion is that the majority of these complexes are physically bound, with very little sharing of electron density; however, for M = Be, and to a lesser extent M = Mg, some evidence for chemical effects is seen in HM(+)-RG complexes involving RG atoms with the higher atomic numbers.