The constant a expresses the strength of the molecular interactions. It has dimension pressure times molar volume squared, [pv ] which is also molar energy times molar volume. The constant b denotes an excluded molar volume due to a finite molecule size, since the centers of two molecules cannot be closer than their diameter, σ σ . A theoretical calculation of these constants for spherical molecules with an interparticle potential characterized by, σ σ , and a minimum energy, − − ε ε , as shown in the accompanying plot produces b = 4 [ 4 π π ( σ σ / 2 ) 3 / 3 ] N A . Multiplying this by the number of moles, N / N A , gives the excluded volume as 4 times the volume of all the molecules. This theory also produces a = I ε ε N A b where I is a number that depends on the shape of the potential function, φ φ ( r ) .