What are NVE, NVT, and NPT?

       In molecular dynamics simulation, the formal statistical mechanic's designations for the ensembles are utilized, so-called "ensemble". They imply the system variables that are regulated or conserved. In all ensembles, the number of particles (N) is conserved.

Visual representation of five statistical ensembles [1]. Generally, the most important ensembles are NVE, NVT, and NPT.

• NVE: Simulation of a system based on constant number (N), constant volume (V), and constant energy (E); 

        the sum of kinetic (KE) and potential energy (PE) is conserved, and T and P are unregulated.

• NVT: Simulation of a system based on the constant number (N), constant volume (V), and constant temperature (T); 

        T is regulated via a thermostat, which typically adds a degree of freedom to the conserved Hamiltonian and P is unregulated.

• NPT: Simulation of a system based on NVT, but the pressure (P) is regulated.

Moreover, other ensembles are used for specific simulations, such as NST and NPH, where "S" is constant-stress and "H" is constant-enthalpy.

Reference

1. https://en.wikipedia.org/wiki/Statistical_ensemble_(mathematical_physics)

Rangsiman Ketkaew