A Newtonian Algorithm for Constant Pressure Molecular Dynamics with Periodic Boundary Conditions*

The concept of constant pressure molecular dynamics (MD) introduced by Andersen forty years ago became a staple of modern MD calculations. Most of the algorithms proposed so far to maintain constant pressure in the course of an MD simulation utilize scaled coordinates to describe particle dynamics. We propose an alternative approach where such dynamics is described by Newtonian equations of motion for particles in their unscaled Cartesian coordinates, as in constant volume simulations.  The MD cell size dynamics, described by Newtonian equations of motion for time-dependent cell vectors, is driven by a balance of the compressive force of the external pressure and the sum of inter-particle forces across MD cell boundary. The performance of the proposed algorithm is verified by test numerical calculations.