which is known as the virial equation, and the components B i ( T ) are the virial coefficients. Each of the virial coefficients corresponds to one term from the cluster expansion ( B 2 ( T ) is the two-particle interaction term, B 3 ( T ) is the three-particle interaction term and so on). Keeping only the two-particle interaction term, it can be shown that the cluster expansion, with some approximations, gives the Van der Waals equation.