Molecular Monte Carlo simulation method of systems connected to three reservoirs

In conventional molecular simulation, metastable structures often survive over considerable computational time, resulting in difficulties in simulating equilibrium states. In order to overcome this difficulty, here we propose a newly devised method, molecular Monte Carlo simulation of systems simultaneously connected to three reservoirs: chemical potential, pressure, and temperature. Both the number of particles and the system size are additional degrees of freedom in such systems. These correspond to additional dimensions of the phase space, which provide shortcuts from the nonequilibrium state to the equilibrium state. Furthermore, these extensive variables are simultaneously and spontaneously tuned in the present method, so that the system automatically reaches the true equilibrium structure, i.e., the equilibrium state in the thermodynamic limit. On the other hand, because of Gibbs-Duhem equation, which indicates that the thermodynamic potential of the systems with the three reservoirs identically vanishes, this ensemble has been considered impossible or physically irrelevant. Here we demonstrate that such systems can be built. We construct thermodynamics and statistical mechanics of this ensemble and design the simulation method.