A coarse-grained molecular dynamics simulation of protrusion of a virtual model lamellipodium by actin polymerization has been performed in order to study the effect of stiffness of the F-actin filament, the G-actin monomer concentration, and the number of polymerization sites on lamellipodium protrusion. The virtual lamellipodium is modeled as a low-aspect-ratio doubly capped cylinder formed by triangulated particles on its surface. It is assumed that F-actin filaments are firmly attached to a lamellipodium surface where polymerization sites are located, and actin polymerization takes place by connecting a G-actin particle to a polymerization site and to the first particle of a growing F-actin filament. It is found that there is an optimal number of polymerization sites for rapid lamellipodium protrusion, which results from competition between the number of polymerization sites and the number of available G-actin particles, and the degree of pulling and holding of the lamellipodium surface by non-polymerized actin filaments. The virtual lamellipodium speed distribution is found to be Maxwellian for particles with random motion in two dimensions, indicating that the actin polymerization process is in thermal equilibrium, resulting in directional lamellipodium motion.