Shell model simulations by adiabatic dynamics

A method is introduced by which molecular dynamics simulations of ionic materials can incorporate the shell model of ionic polarizability. The shells are given a small mass and their motion integrated in the same way as that of the cores, by numerical integration of the classical equations of motion. The authors argue that results should be in agreement with the conventional massless shell model providing that the frequency of the spring linking the core and the shell is much higher than the lattice vibrational frequencies. It is always possible to satisfy this condition by making an appropriate choice of the shell mass. The method is tested in simulations of diffusion in molten NaCl and superionic CaF₂, and of phonon frequencies in MgO. The method works well, is easy to implement, and is computationally efficient.