Molecular dynamics in other ensembles

Standard molecular dynamics simulations simulate the microcanonical ensemble and, with a constant Hamiltonian, the kinetic and potential energies fluctuate with opposite signs (Fig. 16.2). 

The pressure also fluctuates. The instantaneous values can be determined with the following expression  

In general, ifwe want to calculate iVyr otNPT ensemble properties, there is no simple way to conduct NVE simulations and change the energy E and volume V until the temperature and pressure of the system reach desired values. 

We can, however, modify the equations of motion and simulate in the appropriate ensemble. In the following sections, we present some of the most widely used methods for simulating systems at constant temperature and pressure. 

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