Here, δ δ is a positive infinitesimal constant, V q is V eff ( q ) − − V ind ( q ) and f k is the carrier distribution function which is the Fermi–Dirac distribution function for electrons in thermodynamic equilibrium. However this Lindhard formula is valid also for nonequilibrium distribution functions. It can be obtained by first-order perturbation theory and the random phase approximation (RPA).