  The maximum entropy formalism developed by Jaynes determines the relevant
ensemble in nonequilibrium statistical mechanics by maximising the entropy
functional subject to the constraints imposed by the available information. We
present an alternative derivation of the relevant ensemble based on the
Kullback-Leibler divergence from equilibrium. If the equilibrium ensemble is
already known, then calculation of the relevant ensemble is considerably
simplified. The constraints must be chosen with care in order to avoid
contradictions between the two alternative derivations. The relative entropy
functional measures how much a distribution departs from equilibrium.
Therefore, it provides a distinct approach to the calculation of statistical
ensembles that might be applicable to situations in which the formalism
presented by Jaynes performs poorly (such as non-ergodic dynamical systems).
