EMPIRICAL POTENTIAL-ENERGY FUNCTION FOR CALCIUM SOLIDS AND CLUSTERS

An empirical potential-energy function, consisting of two- and three-b ody terms, has been derived for the study of calcium solids and cluste rs. With a single set of parameters, this potential reproduces the pho non frequencies and elastic constants of both the low-temperature fee and the high-temperature bcc crystalline phases to a high degree of ac curacy. It also gives a sensible energy profile for the tetragonal (Ba in path) interconversion of the two structures, which are both minima along the path. The potential predicts that the fee and hcp solids hav e almost equal cohesive energies, with that of the bcc phase being 0.0 2 eV lower. Other cubic phases and various two-dimensional and one-dim ensional structures have lower cohesive energies. The potential leads to the conclusion that the most stable clusters tend to be polytetrahe dral, leading to icosahedral packing, thereby maximizing coordination number.