AN AB-INITIO INVESTIGATION OF STRUCTURE AND ENERGETICS OF CLUSTERS KNCLN AND LINFN

Results of a systematic study of clusters K(n)Cl(n) and Li(n)F(n) up to n = 32 are reported within self-consistent-field (SCF), second-order Moller-Plesset perturbation theory (MP2), and the single- and double-excitation coupled cluster method with a perturbational estimate of triple excitations CCSD(T). Stationary geometries are characterized as local minima by SCF force field calculations up to K9Cl9 and Li8F8, respectively. The clusters K(n)Cl(n) show great similarities with previously studied Na(n)Cl(n). The energetically most stable isomers of even the smaller potassium chloride clusters show a clear preference for geometries which are fragments of the solid state lattice. This effect is enhanced by electron correlation. Lithium fluoride clusters, Li(n)F(n), show a different behaviour: hexagonal structures (stacked six-rings) are prefered to lattice fragments (cubic) on the SCF level at least up to Li18F18. This preference is only slightly reduced by electron correlation. The present results for K(n)Cl(n) and Li(n)F(n) allow for an estimate of lattice energies by extrapolation n --> infinity, which is in good agreement with experiment. The structure constants found for K32Cl32 approach that of the solid. This is different for Li32F32: clusters of LiF show a slower convergence towards bulk properties on increasing cluster size.