FIRST-PRINCIPLES CALCULATION OF THE MG(0001) SURFACE RELAXATION

First-principles calculations for the clean Mg(0001) surface indicate that the first three interlayer spacings are expanded with respect to the bulk value by 1.5%, 0.5%, and 0.1%, respectively. These results ar e in excellent agreement with an analysis of low-energy electron diffr action I-V spectra that yields changes in the first three interlayer s pacings of 1.9%, 0.8%, and -0.4%, respectively [P.T. Sprunger, K. Pohl , H.L. Davis and E.W. Plummer, submitted]. The calculations also show that the outward relaxation is accompanied by a 2.8% decrease in the p sigma population of the surface-layer; atoms (p(sigma) is defined wit h respect to the surface normal). This is somewhat smaller than the 7. 8% decrease reported to accompany the outward relaxation of the clean Be(0001) surface [P.J. Feibelman, Phys. Rev. B 46 (1992) 2532].