NVE

This ensemble involves keeping the number of particles (N), system volume (V) and energy (E) constant / conserved.  It is also called the microcanonical ensemble.  Also see molecular dynamics simulations.

To achieve energy conservation, double precision may be required, and there are several important settings in the .mdp file that should be considered:

1. rlist should be 0.1 - 0.3 nm larger than the longest cutoff (rvdw or rcoulomb); the additional buffer size should be motivated by the distance between charge groups and the diffusion of these groups throughout the system.
2. dt should be reasonable for the system considered.  With constraints, up to 2.0 fs may be appropriate, otherwise a timestep of 0.5 - 1.0 fs may be necessary.
3. The treatment of electrostatics (coulombtype) may also strongly influence energy conservation.  Plain cutoffs are inappropriate.  One should use PME, PME-Switch, or Reaction-Field-zero for good energy conservation.
4. nstlist should be between 5 and 20 steps (or use nstlist = -1 for automated neighborsearching).