Molecular Dynamics at Constant Temperature

Conventional MD simulation conserves total energy; hence, the time averages computed from MD simulation, if it is long enough, are equivalent to ensemble averages computed from the microcanonical ensemble. The flexibility of MD is greatly enhanced by noting that it is not restricted to NVE. There exist techniques by which MD can simulate in the NVT or NPT ensembles as well. We will consider some popular temperature control schemes, and one popular pressure control scheme, in this section.

There are essentially three ways to control the temperature in an MD simulation:

1. Velocity rescaling;
2. Stochastic forces and/or velocities; and
3. “Extended Lagrangian” formalisms.

Each of these classes of schemes has advantages and disadvantages, depending on the application. In the following subsections, we consider several examples of thermostats, and attempt to discuss their advantages and drawbacks. A simple barostat is also described in the last section.