Consider a system of charged particles (not dipoles), obeying classical mechanics and classical statistics. Show that the magnetic susceptibility of this system is identically zero (Bohr--van Leeuwen theorem).
[Note that the Hamiltonian of this system in the presence of a magnetic field H(=∇×A) will be a function of the quantities pj+(ej/c)A(rj), and not of the pj as such. One has now to show that the partition function of the system is independent of the applied field.]