The adaptive resolution simulation (AdResS) technique couples regions with different molecular resolutions and allows the exchange of molecules between different regions in an adaptive fashion. The latest development of the technique allows to abruptly couple the atomistically resolved region with a region of non‐interacting point‐like particles. The abrupt set‐up is derived having in mind the idea of the atomistically resolved region as an open system embedded in a large reservoir at a given macroscopic state. In this work, starting from the idea of open systems, the authors derive thermodynamic relations for AdResS which justify conceptually and numerically the claim of AdResS as a technique for simulating open systems. In particular, the relation between the chemical potential of the AdResS set‐up and that of its reference fully atomistic simulation is derived. The implication of this result is that the grand potential of AdResS can be explicitly written and thus, from a statistical mechanics point of view, the atomistically resolved region of AdResS can be identified with a well‐defined open system.

In a previous paper, a precise relation between the chemical potential of a fully atomistic simulation and the simulation of an open system in the adaptive resolution method (AdResS) was identified. The starting point was the equivalence derived from the statistical partition functions between the grand potentials, Ω, of the open system and of the equivalent subregion in the fully atomistic simulation of reference. In this work, instead, the authors treat the identity for the grand potential based on the thermodynamic relation normalΩ=−pV$\Omega =-pV$ and investigate the behavior of the pressure in the coupling region of the adaptive resolution method (AdResS). The authors confirm the physical consistency of the method for determining the chemical potential described by the previous work and strengthen it further by identifying a clear numerical relation between the potential that couples the open system to the reservoir on the one hand and the local pressure of the reference fully atomistic system on the other hand. Such a relation is of crucial importance in the perspective of coupling the AdResS method for open system to the continuum hydrodynamic regime.

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