THE FUNCTIONAL DEPENDENCE OF THE COHESIVE ENERGY ON COORDINATION IN TRANSITION-METAL SYSTEMS

The energy of an atom as a function of its coordination number has bee n calculated within the tight-binding model for added vacancies in FCC and BCC lattices considering s and d orbitals. We show that the dispe rsion due to the different possible topologies of nearest-neighbour va cancies is larger when d bands are replaced by five degenerate s bands in simplified models. We also show that when calculations are per-for med going beyond the second moment the cohesive band energy follows ap proximately a square-root dependence with coordination for s bands but that for d bands it has a different dependence. Besides the case of a bulk atom with different numbers of nearest-neighbour vacancies, the cases of surfaces and lattices containing uniformly distributed vacanc ies have also been considered.