A coupled cluster study of the stability of lithium clusters

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Abstract
Coupled cluster studies on Li2, on the Li6 ring and on other Li6clusters are reported. In its linear approximation the coupled cluster method gives a larger fraction of the correlation energy for Li2 than the nonlinear version, although other physical properties like force constant and bond length are described unsatisfactory. The planar Li6 ring is predicted to be stable in the equidistant form. Larger rings tend to have a Peierl’s distorted alternant geometry on the Hartree–Fock level. Thus Li behaves somewhat similar to (CH) n , while for H n also the n=6 ring is distorted. The stability of equidistant six‐membered rings is therefore attributed to the existence of rather delocalized 2s electrons. The comparison of the results for Li6clusters of different symmetry (D 6h ,O h ,C 5v ) with similar calculations reported in the literature indicates that the inclusion of p‐functions is essential, whereas the size of the s function subspace is not very important.