The cohesive energy density, V ,is defined as U/V, where U is the mean potential energy of attraction within the sample and V its volume. Show that v= ½ N fV(R)dr, where 91<': is the number density of the molecules and VCR) is their attractive potential energy and where the integration ranges from d to infinity and over all angles. Go on to show that the cohesive energy density of a uniform distribution of molecules that interact by a van der Waals attraction of the form -C61R6 is equal to (2nI3)(N'p/d)M2)P2C6, where p is the mass density of the solid sample and M is the molar mass of the molecules.