A simple analytical description of the non-stationary dynamics in Ising spin systems

The analytical description of the dynamics in models with discrete variables (e.g. Ising spins) is a notoriously di. cult problem, which can only be tackled under some approximation. Recently a novel variational approach to solve the stationary dynamical regime has been introduced by Pelizzola ( 2013 Eur. Phys. J. B 86 120), where simple closed equations are derived under meanfield approximations based on the cluster variational method. Here we propose to use the same approximation based on the cluster variational method also for the non- stationary regime, which has not been considered up to now within this framework. We check the validity of this approximation in describing the nonstationary dynamical regime of several Ising models defined on Erdos-Renyi random graphs: we study ferromagnetic models with symmetric and partially asymmetric couplings, models with random fields and also spin glass models. A comparison with the actual Glauber dynamics, solved numerically, shows that pone of the two studied approximations (the so-called `diamond' approximation) provides very accurate results in all the systems studied. Only for the spin glass models do we find some small discrepancies in the very low temperature phase, probably due to the existence of a large number of metastable states. Given the simplicity of the equations to be solved, we believe the diamond approximation should be considered as the `minimal standard' in the description of the nonstationary regime of Ising-like models: any new method pretending to provide a better approximate description to the dynamics of Ising-like models should perform at least as good as the diamond approximation.