CONVERGENCE OF MULTIBODY EXPANSIONS FOR ALKALINE-EARTH METALS - CONTRAST BETWEEN MAGNESIUM AND BERYLLIUM CLUSTERS

An ab anitio SCF-LCAO-MO study of nonadditive three- and four-body interaction energies in Mg3 (equilateral and linear) and Mg 4 (tetrahedral) clusters is presented. The relative importance of these few-body terms to the interaction energy is at most 39% for three- and 8% for four-body terms. This lies in marked contrast with the case of Be clusters where these terms can have a relative weight of up to 76%, and 31.6%, respectively. The reason for such radically different nonadditive effects is discussed. Some new results for Be3, Be4, and Be 5 clusters are presented confirming this is a case where multibody expansions are expected to be very slowly convergent. Finally these results provide an explanation for the poor convergence of the chemisorption energy of an H atom as a function of the size of Ben clusters. © 1979 American Institute of Physics.