Physics:Isoenthalpic–isobaric ensemble

Short description: Statistical-mechanical ensemble

The isoenthalpic-isobaric ensemble (constant enthalpy and constant pressure ensemble) is a statistical mechanical ensemble that maintains constant enthalpy [math]\displaystyle{ H \, }[/math] and constant pressure [math]\displaystyle{ P \, }[/math] applied. It is also called the [math]\displaystyle{ NPH }[/math]-ensemble, where the number of particles [math]\displaystyle{ N \, }[/math] is also kept as a constant. It was developed by physicist H. C. Andersen in 1980. The ensemble adds another degree of freedom, which represents the variable volume [math]\displaystyle{ V \, }[/math] of a system to which the coordinates of all particles are relative. The volume [math]\displaystyle{ V \, }[/math] becomes a dynamical variable with potential energy and kinetic energy given by [math]\displaystyle{ PV \, }[/math]. The enthalpy [math]\displaystyle{ H=E+PV \, }[/math] is a conserved quantity. Using isoenthalpic-isobaric ensemble of Lennard-Jones fluid, it was shown that the Joule–Thomson coefficient and inversion curve can be computed directly from a single molecular dynamics simulation. A complete vapor-compression refrigeration cycle and a vapor–liquid coexistence curve, as well as a reasonable estimate of the supercritical point can be also simulated from this approach. NPH simulation can be carried out using GROMACS and LAMMPS.