[gmx-users] Use NVT to mimic NVE
baptista at itqb.unl.pt
Sat Oct 11 01:40:27 CEST 2014
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:
>> 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.
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.
>> Thank you.
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