Ensembles in Molecular Modelling

The molecular modelling of systems consisting of many molecules is the field of statistical mechanics, sometimes called statistical thermodynamics [28,29], Basically, the idea is to go from a molecular model to partition functions, and then, from these, to predict thermodynamic observables and dynamic and structural quantities. As in classical thermodynamics, in statistical mechanics it is essential to define which state variables are fixed and which quantities are allowed to fluctuate, i.e. it is essential to specify the macroscopic boundary conditions. In the present context, there are a few types of molecular systems of interest, which are linked to so-called ensembles. 

The relative importance of fluctuations, e.g. in the membrane area, as is possible in a (N, p, y, T) ensemble, becomes larger when the system that is under investigation is smaller. For macroscopic systems, the fluctuations in such quantities are negligible, but, for some of the modelling techniques that are restricted to small systems, one cannot avoid dealing with it. Of course, for a (N, p, y, T) ensemble in which both the volume and the surface area are allowed 

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