Isobaric-isoenthalpic ensemble

Generally, MD is performed to simulate a continuous phase trajectory in the microcanonical N,E,V) ensemble, while in MC method, individual phase points of an (N,V,T) ensemble are simulated. As far as the equilibrium properties are concerned, the trajectory average of MD and the configurational average of MC are equivalent (Allen and Tildesley, 1987). Recently, MD simulation of other types of ensembles also have been achieved. In the N,E,V) simulations, volume and total energy are held constant and the temperature and pressure are allowed to fluctuate. Andersen (1980) suggested methods for simulation of isobaric-isoenthalpic and isobaric-isothermal (A, P,7) ensembles. A... 

Andersen (4) has shown that the average of any function, F, over the trajectory thus generated is equal to the ensemble average for the isoenthalpic isobaric ensemble, i.e.,... 

Winkler, Morawitz, and Yoon reported MD simulations at constant pressure in a recent paper.The volume of the (periodic) box changed while the external pressure was kept constant the two parameters were related by a stress tensor. The authors derived a new instantaneous external stress tensor that allowed both volume and internal pressure to fluctuate during the course of the simulation. The technique did not require the use of artificial parameters or the extension of the phase space sampled. Time-averaged ensembles of various system properties were shown to be comparable to averages from isoenthalpic-isobaric ensembles. 

This is the conceptual framework of the present technique. It is a completely general ab initio VCS-MD which can be used to simulate solids at finite temperatures and pressures. Subjected to small modifications, and in the limit of isotropic fluctuations, it maps into Andersen s dynamics[17], whose averages over trajectories correspond to those of the isoenthalpic isobaric (N,P,H) ensemble. 

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