Dear all,
In the couette flow code it’s used fix 1 all nve. But since the upper wall has a velocity, work was applied to move it, which makes the system not conservative anymore.
Then why nve is applied to all?
Thanks!
Using fix nve does NOT automatically mean that you are in an NVE ensemble.
If you do any manipulation of the system that moves energy in or out, or if you don’t have periodic boundary conditions, or if you add a thermostat fix, and so on, you are not in that ensemble anymore. The fix just implements a plain velocity verlet time integration.
The logic only works the other way around, i.e. if you want to do a simulation of an NVE
ensemble, you must use fix nve (and have no other system manipulation).
The same is true for fix nvt, npt, or nph and the corresponding ensembles, too.
1 Like
It’s useful to think of (for example) fix npt as an NPT integrator which incorporates a thermostat and a barostat.
A macroscopic thermostat cannot keep a house at thermal equilibrium under sufficiently large inflows or outflows of heat – similarly, an NVE integrator cannot (and is not meant to) maintain an NVE ensemble when used in a simulation alongside external heat or work sources.
The NVE integrator may well still have other desirable analytical properties – in particular, thermostats and barostats (by necessity) couple particles’ motions globally and may leave unphysical artifacts in estimated dynamical properties.
1 Like