Ergodicity and convergence of fluctuations in Parrinello-Rahman molecular dynamics

Distortion and rotation of a molecular dynamics cell used in Parrinello-Rahman molecular dynamics are found to lead to slow convergence, or nonconvergence of fluctuations from thermodynamic averages. The variations are shown to be related to nonconservation of the total angular momentum and translational symmetry variance of the dynamics. A modified equation of motion is presented which eliminates these variations. It is shown that the ergodicity is achieved in the MD ensemble generated by the new equations of motion. However, the rate of convergence is strongly affected by the choice of the MD cell mass W. Simulation results show that not all values of W can be used to give a desired convergence of fluctuations from thermodynamic averages in finite simulations. The fastest convergence is achieved by using the optimal cell mass.