The elementary units of granular materials are solid ``mesoscopic'' grains, interacting on contact. The surface is in general rough on a microscopic scale and solid friction is usually found. Here, we focus on properties of granular systems in the absence of friction. We will examine in how far phenomena like stress chains and arching depend on friction by neglecting solid friction. However, we have some kind of ``geometrical friction'', since the particles restrict the motion of their neighbors due to excluded volume effects.
Without friction, energy may still be dissipated by e.g. viscous deformations, modelled here by a simple viscous dashpot, active during the contact.
Since we are interested in static arrangements of particles in the gravitational field, we use strong viscous damping, in order to reach the steady state quickly. For the relaxation of the array we use a molecular dynamics (MD) procedure [18, 19], in order to allow contacts to break. The MD method is not the best choice for a fast relaxation, but closing and opening of contacts is implemented straightforwardly.