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...example when talking about magnetic susceptibility of solids where each atom or ion has a closed shell of electrons with high excitation energy, you get diamagnetism with the susceptibility given by chi=-Ze^2*N/(6mc^2)r^2 Z is the number of electrons in the atom, N is number of atoms per unit volume, r^2 is mean square radious of electron charge cloud

that's all good and well, but the mention that the derivation comes from the "first-order perturbation energy <0|e^2/2mc^2*A^2|0> where the 0s denote ground states and A is the magnetic vector potential

now from what little I know of pertubation theory, to get the first order energy correction you do <0|H'|0> where H' is the perturbed Hamiltonian and |0> is the unpertubed ground states. So the equation in the derivation in the preceding paragraph is vaguely of that form, but I don't get how e^2/2mc^2 * A^2 is the perturbed Hamiltonian