Abstract
We derive an exact expression of the response function to an infinitesimal magnetic field for an Ising–Glauber-like model with arbitrary exchange couplings. The result is expressed in terms of thermodynamic averages and does not depend on the initial conditions or on the dimension of the space. The response function is related to time-derivatives of a complicated correlation function and so the expression is a generalization of the equilibrium fluctuation–dissipation theorem in the special case of this model. Correspondence with the Ising–Glauber model is discussed. A discrete-time version of the relation is implemented in Monte Carlo simulations and then used to study the ageing regime of the ferromagnetic two-dimensional Ising–Glauber model quenched from the paramagnetic phase to the ferromagnetic one. Our approach has the originality to give direct access to the response function and the fluctuation–dissipation ratio.