MODEL DESCRIPTION OF TRANSITION-METALS USING THE ROTATED 2ND MOMENT APPROXIMATION

An interatomic potential is described, the rotated second moment appro ximation (RSMA), which incorporates directional bonding through energy moments evaluated over directional atomic orbitals. When non-directio nal orbitals are used, RSMA reduces to the standard SMA, and is thus c apable of describing metallic systems. A model RSMA potential is demon strated for 3d transition metals, with only first neighbor shell inter actions, which can correctly predict the experimental trend in the rel ative stability of the fcc and hcp structures. A generalization of RSM A is proposed that is related in the same way that the embedded atom m ethod (EAM) is related to the SMA. A generalized fcc RSMA potential is constructed for which the Cauchy pressure can be made negative by sys tematically adjusting one parameter. This behavior results from the an gular (directional) forces and not from a physically-unreasonable embe dding function.