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SUMMARYWe explore two widely used algorithms for uid reservoirs in molecular simulations and demonstrate that they may induce non-physical non-equilibrium e ects, even in systems that should be at equilibrium. For example, correlations of momentum and density uctuations lead to a bias in the mean uid velocity when measured as the mean over samples of instantaneous uid velocity. The non-physical behaviour is entirely computational in origin and is an instance of a more general issue in molecular simulations: a failure to correctly model stochastic properties may induce non-equilibrium behaviour that does not exist in the corresponding physical system. Finally, we demonstrate that simple algorithm corrections eliminate this artifact.

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