[gmx-users] Use NVT to mimic NVE

[Antonio Baptista]

On Fri, 10 Oct 2014, Mark Abraham wrote:

> On Fri, Oct 10, 2014 at 7:41 PM, Johnny Lu <johnny.lu128 at gmail.com> wrote:
>
>> Hi.
>>
>> Is it a good idea to mimic NVE by a NVT simulation with a large temperature
>> coupling time constant, to reduce the effect of the thermostat ?
>>
>
> Not if your observable is the total energy. But for most interesting
> observables, this is more than fine.
>
>
>> If I use V-rescale thermostat, what artifacts will the simulation get if I
>> use a large coupling time (say, 500 ps) and single precision gromacs ?
>>
>
> Probably nobody knows :-) Thermodynamic ensemble differences disappear as
> the system becomes large. But quantifying that from converged ensembles for
> a given observable is work.
>
> Mark

Note also that, unlike averages, fluctuations are ensemble-dependent. 
Although v-rescaling (as most other thermostats) does indeed sample from 
the NVE ensemble in the mathematical limit of infinite coupling constant, 
this does not imply that, in practice, fluctuations converge to those in 
the NVE ensemble for large but finite coupling constants.

As you can check in the v-rescaling paper, fluctuations were found to be 
very robust to changes of the coupling parameter (at least for the kinetic 
and potential energies). This is generally a good thing, but maybe not 
what you want to run a "fake" NVE simulation as you described. So, if you 
are interested in the fluctuations of some properties, you will have to 
check what is in practice their asymptotic behavior.

Best,
Antonio


>
>
>> Thank you.
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