Correllation with van der Waals constants

In the classical van der Waals expression (Equation (64)) we considered the gas molecules to be hard spheres of diameter, 7. If we write this expression in terms of the molecular parameters of a gas then we have 

Vd = OO for r a. For two dissimilar molecules, the London dispersion interaction coefficient Cd, from Equation (68), is 

On the other hand, we may also use gas constants, a and b to calculate the dispersion pair potential. As we will see its derivation in Section 3.4.2, the van der Waals constant, a, can be expressed as 

We know that the constant b is the volume of the rigid molecules (dm3 mob1), which is the volume unavailable for the molecules to move in, and the excluded volume per mole = 4/3 so3 because the spherical diameter, o, is the closest that one molecule can approach another. Thus the constant, b, can be written in terms of molecular diameter so that (see also Section 3.4) 

Relative magnitud es of van der Waals interactions between two identical molecules at 0°C  

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