Fitting of accurate interatomic pair potentials for bulk metallic alloys using unrelaxed LDA energies

We present a general and simple method for obtaining accurate, local densit y approximation (LDA-) quality interatomic potentials for a large class of bulk metallic alloys. The method is based on our analysis of atomic relaxat ion, which reveals that the energy released in the relaxation process can b e approximated by calculating the epitaxially constrained energies of the c onstituents A and B. Therefore, the pair potential is fitted to the LDA-cal culated epitaxial energies of the constituents (to capture the relaxation e nergies), and to the unrelaxed energies of ordered A(n)B(m) compounds (to c apture the fixed-lattice "chemical" energy). The usefulness of our approach is demonstrated by carrying out this procedure for the Cu1-xAux alloy syst em. The resulting pair potential reproduces the relaxed LDA formation energ ies of ordered compounds rather accurately, even though we used only unrela xed energies as input. We: also predict phonon spectra of the elements and ordered compounds in very good agreement with the LDA results. From the cal culations for approximate to 10 000 atom supercells representing the random alloy, we obtain the bond lengths and relaxation energies of the random ph ase that are not accessible to direct LDA calculations. We predict that, wh ile in Cu-rich alloys the Cu-Cu bond is shorter than the Cu-Au bond, at hig her Au compositions this order is switched. Furthermore, we find that Au-ri ch Cu1-xAux alloys have ground states that correspond to (001) superlattice s of n monolayers of fee Au stacked on m monolayers of the L1(0) CuAu-I str ucture. The potential developed in this work is available at the site http: //www.sst.nrel.gov/data/download.html for interested users.