The problem of the detaching shell in the shell model potential for oxides

Many sets of shell model parameters for oxides suffer from the defect that when the anion–cation pair is not restrained by the normal lattice forces, the ions collapse into each other. This can degrade their usefulness in the study of, for example, solid surfaces where normal lattice restraining forces are missing. We discuss adding a higher-order constraining force to the core–shell interaction in the failed models. The effects on the relative permittivities and elastic constants suggest a general reparameterization, in conjunction with standardized higher-order constraining forces, is needed. This constitutes an extension of the shell model. The ensuing models should acquire more realistic repulsive forces at short range.