Internal stress tensor in constant-pressure molecular dynamics of anisotropic molecular solids

We introduce a new procedure to obtain the internal stress tensor in constant-pressure molecular dynamics (MD) simulations of molecular solids. We show that the internal stress tensor which governs the dynamics of the MD cell depends not only on the center-of-mass positions and velocities of the molecules, but also explicitly on the molecular orientations and angular velocities. The importance of these rotational contributions in a two-dimensional diatomic molecular monolayer system is discussed.