If all the forces, which appear in the Newton equation of motions, are related to the potential energy of the system, then the total energy of the system E=Ekin+Epot is conserved. If the total number of atoms N and the volume V (of the unit cell) are also kept constant, then the MD simulations are said to be performed in the microcanonical (NVE) ensemble. Generally, if the simulation system is sufficiently large, the small part of it may be considered as a canonical system. For large NVE systems the fluctuations in temperature are small, and it may be considered approximately constant. There are situations, in which temperature must be kept constant. For example, studying temperature induced unfolding of proteins requires precise temperature control. Therefore, for these classes of problems MD must reproduce an isothermal ensemble, such as canonical NVT ensemble, in which the number of particles, volume, and temperature are fixed.
An ensemble with a constant number, N, of particles in a constant volume, V, and thermal equilibrium with a heat bath at constant temperature, T. The energy dependence of probability density confirms to the Boltzmann distribution. This distribtion is called the canonical ensemble.
The isothermal-isobaric ensemble is a statistical mechanical ensemble that maintains a constant total number of particles, and constant temperature (T) and pressure (p), typically abbreviated NpT. This ensemble plays an important role in chemistry since the majority of important chemical reactions are carried out under constant pressure conditions. The isothermal-isobaric ensemble is also important when attempting to describe the Gibbs free energy of a system, which is the maximum amount of work a system can do at constant pressure (p) and temperature (T).