  We derive a microscopic expression for the dielectric susceptibility $\chi$
of a Coulomb glass, which corresponds to the definition used in classical
electrodynamics, the derivative of the polarization with respect to the
electric field. The fluctuation-dissipation theorem tells us that $\chi$ is a
function of the thermal fluctuations of the dipole moment of the system. We
calculate $\chi$ numerically for three-dimensional Coulomb glasses as a
function of temperature and frequency.
