Simulations of a droplet impacting a flat solid surface with a small initial speed have been studied using molecular dynamics. Approximating the shape of the drop by a spheroid, spreading radii, and dynamic contact angles are measured. The data reproduce well experimental results from literature. We show that the difference between the equilibrium and the dynamic contact angle cosines, that is, the spontaneous driving force, versus the spreading velocity of the three-phase line varies with impact speed and consists of two distinct regimes which can be described by existing models of moving contact lines.