Onset of convection in molecular dynamics simulations of grains

We investigate granular materials undergoing vibrations, using a molecular dynamics (MD) algorithm, and we display limitations of the numerical method. We show that previously reported convection patterns obtained through MD may be due to the microscopic interactions. MD calculations often use parameters which lead to unphysically large contact times between beads and to large density fluctuations. These in turn enhance the appearance of convection rolls, a numerical finding without experimental counterpart.