Dynamics with the Shell Model

Abstract

Potentials for ionic materials are among the best classical models currently available, giving very realistic descriptions of many physical properties. The key feature in providing this realism has been found to be the inclusion of ion polarisability, most often via the shell model. Until recently though, the dynamical properties of such models, and their comparison with experiment, have received little attention due to the computational workload involved in dealing with the extra degrees of freedom.

We discuss here recent progress in the dynamical simulation of ionic materials using shell-model potentials. This includes the approaches which have successfully been developed to deal with the central problem, which is the requirement that the system obeys the Born-Oppenheimer principle with respect to the polarisation. The results of extensive simulations of UO₂, using the best potentials available, and comparison with neutron-scattering and other experimental data reveal the feasibility of shell-model MD, and also shed new light on the capabilities of the potentials.

Key Words:

• Shell-model potentials
• Ionic materials
• Molecular dynamics
• conjugate gradients
• superionic conduction
• melting