One problem with the determination of entropy is the difficulty of measuring heat
capacities near T = 0. There are good theoretical grounds for assuming that the heat
capacity is proportional to T3 when T is low (see Section 7.1), and this dependence is
the basis of the Debye extrapolation. In this method, Cp is measured down to as low a
temperature as possible, and a curve of the form aT3 is fitted to the data. That fit determines
the value of a, and the expression Cp,m = aT3 is assumed valid down to T = 0.