# temperature in NVE ensembles

Hi everybody,

I've got a basic question I think. I'd like to create several different
NVE ensembles in a simple system (say a crystal sylicon, or LJ Argon).
Temperature in equilibrium is not a well defined quantity in such ensembles but
oscillates around a mean value T , that depends in a non trivial way on the
total energy, and that we call the 'temperature' of the system.

The point is that I'd like to run different NVE simulations fixing a priori
this mean value of the temperature. Which is the best efficient way in lammps
to achieve that ? (equilibrating with an NVT thermostat and than switch to NVE
gets me close to the right temperature but not close enough)

Maybe it sounds strange to work in a NVE ensemble and to fix the mean value of
the temperature, but this is what I'd like to do...

Hi ar_ma86,

I am not sure what you are trying to do but, one of the ways could be as following.
Run a set of your simulations at different volumes and what temperature you get for a certain volume in NVE. Fit that curve to some good functional form, and back calculate the volume that give you a certain temperature and run your NVE at that temperature.

Hope this helps.

Hi everybody,

I've got a basic question I think. I'd like to create several different
NVE ensembles in a simple system (say a crystal sylicon, or LJ Argon).
Temperature in equilibrium is not a well defined quantity in such ensembles but
oscillates around a mean value T , that depends in a non trivial way on the
total energy, and that we call the 'temperature' of the system.

The point is that I'd like to run different NVE simulations fixing a priori
this mean value of the temperature. Which is the best efficient way in lammps
to achieve that ? (equilibrating with an NVT thermostat and than switch to NVE
gets me close to the right temperature but not close enough)

what is close enough? if you run your simulation long enough with a
proper thermalization until it is well equilibrated, then it should
maintain that (average) temperature. if not, then you need to
investigate your settings, how long you ran the MD, how well you
sampled phase space, how well it is equilibrated, how well you
conserve energy. when running with a thermostat reduce the intensity
of the thermal coupling, you should transition from NVT to NVE
ensemble.

Maybe it sounds strange to work in a NVE ensemble and to fix the mean value of
the temperature, but this is what I'd like to do...

why is it not sufficient to compute the temperature of your system
after the fact and use just whatever value you get? that is what
people have done for 30+ years when running NVE simulations.

axel.