The difference in this equation from classical Chapman–Enskog theory lies in the streaming operator S i j , within which the velocity distribution of the two particles are evaluated at different points in space, separated by σ σ i j k , where k is the unit vector along the line connecting the two particles centre of mass. Another significant difference comes from the introduction of the factors g i j , which represent the enhanced probability of collisions due to excluded volume. The classical Chapman-Enskog equations are recovered by setting σ σ i j = 0 and g i j ( σ σ i j k ) = 1 .