For the van der Waals interaction with potential-energy function given by Eq. (13.25), show that when the magnitude of the displacement x from equilibrium (r = Ro) is small, the potential energy can be written approximately as U ½ kt2 – U0 [Hint: In Eq. (13.25), let r = Ro + x and u = x/R". Then approximate (I + u)" by the first three terms in Eq. (13.28).1 How does k in this equation compare with the force constant in Eq. (13.29) for the force?