Show that the mean interaction energy of N atoms of diameter d interacting with a potential energy

Question:

Show that the mean interaction energy of N atoms of diameter d interacting with a potential energy of the form C61R6 is given by U=-2NzC6/3Vd3, where V is the volume in which the molecules are confined and all effects of clustering are ignored. Hence, find a connection between the van der Waals parameter a and C6, from nZalV2= (∂U/∂V)T