Effective simulations of the electrochemical double layer, especially at low ionic concentrations, would require treating open systems in an out-of-equilibrium state in which particles are exchanged in a similar fashion as in the Grand Canonical ensemble.
A similar problem is encountered while studying nucleation and crystal growth in heterogeneous solutions, in which the driving force depends on the composition of the fluid.
In this talk, I will outline two different simulation methods to tackle equilibrium and out-of-equilibrium open systems. I will discuss the founding principles and the practical implementation of the Hamiltonian Adaptive Resolution Scheme (H-AdResS) for aqueous solutions. This approach, for which we have developed a solid statistical mechanics foundation, allows one to couple a high-resolution region, which can be treated quantum mechanically, to a lower-resolution region. The latter can be exploited as a reservoir of particles, thus effectively making the high-resolution region grand canonical. However, practical implementation issues have made this approach computationally ineffective and difficult to maintain in MD software packages. To circumvent these issues, we have recently developed a method to enforce a pseudo-grand-canonical ensemble in solutions through a membrane with harmonic volumetric restraints to control the solute osmotic pressure. The efficacy of this approach is tested on the case of ice growth from a NaCl solution.
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