Nonlinear response theory: Transport coefficients for driving fields of arbitrary magnitude

A theory of nonlinear response is developed for driving fields of arbitrary magnitude. Exact and usable expressions are provided for electrical and thermal mobility, and related transport coefficients, in terms of correlation functions of the system. A generalization into the nonlinear domain is provided of the Wiedemann-Franz law connecting electrical and thermal response and of the Einstein relation relating the diffusion constant and the mobility.