Interstitial-electron model for lattice dynamics in fcc metals

We propose and test the interstitial-electron model (IEM) for lattice dynamics in close-packed structures. The IEM model treats the valence electrons as classical lattice particles localized at interstitial tetrahedral positions, as suggested by the ab initio generalized-valence-bond cluster calculations of McAdon and Goddard. We apply the IEM to the fcc metals Ni, Pd, Pt, Ag, Au, and Cu using a simple six-parameter description (nearest-neighbor electron-electron, electron-ion, ion-ion terms, each with two parameters) to exactly fit lattice constants, elastic constants (C₁₁,C₁₂,C₄₄), and the two lattice modes at the X point in the first Brillouin zone. The predicted phonon-dispersion relations are in excellent agreement with experiment for all branches in the high-symmetry [100], [110], and [111] directions. The explicit inclusion of valence electrons in the interparticle interactions implicitly includes what would be considered as many-body effects in the usual ion-ion scheme (e.g., C₁₂{≠}C₄₄). Such force fields should also be useful for describing nonperiodic systems (surfaces, clusters, and defects).