In gases molecular rearrangements are stochastic processes, that is, the relative amount of each configuration is determined by the difference in free energy between the molecular configurations. In normal liquids the same relationships apply, although AG is perturbed by differences in nearest-neighbour interactions with each configuration. In a strongly bound solid it may be that no molecule can transform until they all do, in this case the process involves a first-order phase transition. In weakly bound molecular crystals, polymers and similar media, the range of interaction may lie between the nearest-neighbour and the macroscopic limit, that is, the process may be neither first order nor stochastic.