Measuring the fluctuation-dissipation ratio in glassy systems with no perturbing field.

A method is presented for measuring the integrated linear response in Ising spin system without applying any perturbing field. Large-scale simulations are performed in order to show how the method works. Very precise measurements of the fluctuation-dissipation ratio are presented for three different Ising models: the two-dimensional ferromagnetic model, the mean-field diluted three-spin model, and the three-dimensional Edwards-Anderson model.