The presence of a confining boundary can change the local structure of a liquid markedly. However, also open boundaries can modify the outcomes of observations made on a sample that is virtually cut out from a large piece of homogeneous liquid. I will show that local den- sity fluctuations pick up finite-size corrections in the presence of a surface with free bound- ary conditions [1]. The analytic expressions, in excellent agreement with simulation data, show that the effect is particularly pronounced if the size of the open sub-volume decreases below the correlation length. This dependence can be used to establish a finite-size scaling analysis based on open subsystems for the calculation of critical points in the phase diagram of, e.g., a binary liquid [2]. To this end, it is necessary to account for two competing length scales: the extent of the sub-volume and the size of the simulation box. Eventually, I will discuss molecular dynamics simulations far from equilibrium by coupling an open system to thermodynamically distinct reservoirs [3]. We have used this approach to study steady heat and mass flows in liquid samples.
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