In the basic MD setting the macroscopic observables are
held constant (where is the total linear momentum.)
This defines the so-called ``MD ensemble'' which can be shown to be
thermodynamically equivalent to the other ensembles.
Other ensembles may be of advantage for specific problems:
is useful for comparisons with theory and MC simulations;
non-equilibrium simulations require a thermostat, thus ;
phase transitions are best studied in an ensemble, etc.