Leapfrog Rotational Algorithms for Linear Molecules

Abstract

A study is made of four algorithms which integrate the rotational equations of motion for rigid linear molecules. They are leapfrog algorithms in the sense that the quantities saved between time steps are the on-step orientation and the mid-step angular velocity. Thermostatted versions of the algorithms as well as conventional energy-conserving versions are described. The algorithms are extensively tested in simulations of liquid nitrogen, the aim being to study the effect of increased time steps on a range of measured properties. The most successful algorithm, based on applying a lenght constraint to the axis vector, shows remarkable stability and can be used with very large time steps.