Entropy and Susceptibility of a Pure Dipole-Dipole System (Cerous Magnesium Nitrate)

The Van Vleck moment expansion is applied to a pure dipole system. On the basis of the long-range nature of the forces, sequences of diagrams are selected that give the dominant contributions in the lattice sums. A selection of diagrams contributing to the entropy and susceptibility is given. There are three different summations to be performed for each type of diagram: the trace over the spin variables, the lattice summation, and the summation over the Cartesian coordinates. The second is performed on the computer and the last by means of the Kramers-Wannier diagonalization. In order to obtain the contributions of diagrams of higher order a Fourier transform is performed. The calculations were performed for Cerous Magnesium Nitrate using a g factor that is zero along the c axis.