The Noro–Frenkel law suggests to condensate the three quantities which are expected to play a role in the thermodynamics behavior of a system (hard-core size, interaction energy and range) into a combination of only two quantities: an effective hard core diameter and the reduced second virial coefficient. Noro and Frenkel suggested to determine the effective hard core diameter following the expression suggested by Barker based on the separation of the potential into attractive V att and repulsive V rep parts used in the Weeks–Chandler– Andersen method. The reduced second virial coefficient, i.e., the second virial coefficient B 2 divided by the second virial coefficient of hard spheres with the effective diameter can be calculated (or experimentally measured) once the potential is known. B 2 is defined as