# R: Re: temperature in NVE ensembles

----Messaggio originale----
Da: [email protected]
Data: 13/08/2013 15.02
A: "[email protected]..."<[email protected]...>
Cc: "LAMMPS Users Mailing List"<[email protected]>
Ogg: Re: [lammps-users] 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)

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.

I'll try to check this. What I observed till now is that if I try to
equilibrate using NVT at a certain temperature T and after equilibration I
switch to NVE, temperature will start oscillating aroung a value close to T but
not exactly T (say a 10% difference). Maybe I did something wrong I'll try to
check once again the equilibration...

I didn't understand your last sentence:

"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.

Because I'd like to study different replicas of the same system.

----Messaggio originale----
Da: [email protected]
Data: 13/08/2013 15.02
A: "[email protected]..."<[email protected]...>
Cc: "LAMMPS Users Mailing List"<[email protected]>
Ogg: Re: [lammps-users] 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)

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.

I'll try to check this. What I observed till now is that if I try to
equilibrate using NVT at a certain temperature T and after equilibration I
switch to NVE, temperature will start oscillating aroung a value close to T but
not exactly T (say a 10% difference). Maybe I did something wrong I'll try to
check once again the equilibration...

I didn't understand your last sentence:

"when running with a thermostat reduce the intensity
of the thermal coupling, you should transition from NVT to NVE
ensemble."

if you increase the coupling constant (or characteristic time) of the
nose-hoover, it will couple less. essentially, you control with it the
characteristic frequency that you couple to. if this is very low, the
exchange of kinetic energy between the nose-hoover chains and your
system will be very slow and at some point there should not be a
significant difference.

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.

Because I'd like to study different replicas of the same system.

but how does that require to run at a that narrowly defined temperature.

also, have you checked for finite size effects? perhaps your system is
simply too small.

axel.