The isoenthalpic-isobaric ensemble (constant

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

and constant

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

ensemble) is a statistical mechanical

ensemble Ensemble may refer to: Art * Architectural ensemble * Ensemble (album), ''Ensemble'' (album), Kendji Girac 2015 album * Ensemble (band), a project of Olivier Alary * Ensemble cast (drama, comedy) * Ensemble (musical theatre), also known as the ...

that maintains constant enthalpy

H \,

and constant pressure

P \,

applied. It is also called the

NPH

-ensemble, where the number of particles

N \,

is also kept as a constant. It was developed by physicist H. C. Andersen in 1980. The ensemble adds another

degree of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

, which represents the variable volume

V \,

of a system to which the coordinates of all particles are relative. The volume

V \,

becomes a dynamical

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

with potential energy and

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...

given by

PV \,

. The enthalpy

H=E+PV \,

is a

conserved quantity In mathematics, a conserved quantity of a dynamical system is a function of the dependent variables, the value of which remains constant along each trajectory of the system. Not all systems have conserved quantities, and conserved quantities are ...

. Using isoenthalpic-isobaric ensemble of Lennard-Jones fluid, it was shown Kioupis, L. I.; Arya, G.; Maginn E. I. Pressure-enthalpy driven molecular dynamics for thermodynamic property calculation II: applications. ''Fluid Phase Equilibria'' 200, 93–110 (2002

/ref> that the Joule–Thomson coefficient and inversion curve can be computed directly from a single

molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of t ...

simulation. A complete vapor-compression

refrigeration cycle Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that allows for the transmission of heat from one locat ...

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 Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is a molecular dynamics program from Sandia National Laboratories. LAMMPS makes use of Message Passing Interface (MPI) for parallel communication and is free and open-source softw ...

References

Statistical ensembles {{statisticalmechanics-stub