In computer simulations of granular flow, one widely used technique is classical soft-sphere Molecular Dynamics, where the equations of motion of the particles are numerically integrated. This requires specification of the forces acting between grains. In this paper, we systematically study the properties of the force laws most commonly used and compare them with recent experiments on the impact of spheres. We point out possible problems and give criteria for the right choice of parameters. Finally, two generic problems of soft-sphere simulations are discussed.