It is convenient to suppose that two molecules exert a negligible effect on each other unless their center of gravity approach within a critical distance "b". A collision therefore begins when the respective centers of gravity arrive at this critical distance, and is completed when they again reach this critical distance on their way apart. Under this model, a collision is completely described by the matrix formula_3, which refers to the constellation ("i", "j") before the collision, and the (in general different) constellation ("k", "l") after the collision.