Surface structure of MgO (001): Ab initio versus shell model

We have performed first-principles calculations on quasicubic MgO clusters for sizes up to 64 atoms. The results of these calculations have been used to determine a generalized environmentally dependent shell model (EDSM). In addition to reproducing the results from the first-principles calculations we suggest that the resulting EDSM parameters are more physical and more transferable than previous parametrizations based on bulk properties. First, we show that the force-constant matrix derived from the EDSM leads to an efficient approach to preconditioning. A full geometrical optimization of the quasicubic 64-atom MgO cluster is accomplished with a total of four first-principles force evaluations which is significantly less than what is required from a conjugate-gradient algorithm. Second, we use the shell-model parameters to study the relaxation and rumpling of an infinite (001) surface. The agreement between the EDSM and recent medium-energy scattering results is excellent and is significantly better than the agreement between conventional shell-model results and experiment. We show that these improvements are due to a more accurate accounting of the appropriate atomic polarizabilities: not only are these insensitively determined by previous methods, but also those for surface atoms are quite different from their counterparts for bulk atoms.