[gmx-users] Pressure Question

Johnny Lu johnny.lu128 at gmail.com
Thu Nov 6 12:39:34 CET 2014

I was not offended by the suggestion. With a sufficiently large number of
water molecules, the protein would behave in the same way, under all three
Barostat and thermostat are artificial in some way. At least, even the
diffusion coefficient is different in a force field paper that used both
NVE and NVT. Langevin thermostat destroys some momentum transfer.

This is my first paper, and I don't know if the reviewer will be fine if I
report that my simulation has an average pressure of 11 bar with error 0.5

On Thu, Nov 6, 2014 at 5:34 AM, Téletchéa Stéphane <
stephane.teletchea at univ-nantes.fr> wrote:

> Le 06/11/2014 06:16, Antonio Baptista a écrit :
>> In particular, the virial-based "instantaneous pressure" (call it P')
>> computed in simulations has its ensemble average equal to the thermodynamic
>> pressure P (check any good book on molecular simulation). But, as others
>> already pointed out, this P' is well-known to show extremelly large
>> fluctuations, meaning that its average computed from the simulation has
>> usually a very large statistical spread. In other words, although the
>> ensemble average of P' is strictly equal to P, its simulation average is a
>> random variable that often shows large deviations from P (especially for
>> short simulations). To get an idea of what is an acceptable error for the
>> average of P', you may look at its distribution histogram in the NPT
>> simulation.
> Dear Antonio,
> Sorry if my message sound "aggressive" when I talked about "totally
> irrevelevant", I will clarify my thoughts.
> From a theoretical point of view, you are right, each ensemble is
> accessible.
> From a biological point of view, though, the concept of fixing the volume
> is less reasonable:
> we live at constant pressure and temperature, and also at tighly
> controlled pH, and salt concentrations.
> The volume varies though, as you feel it when the weather is getting hot
> or cold.
> My point was exactly what your are telling in a more formal way than me:
> "this P' is well-known to show extremely large fluctuations"
> Well, digging a bit more on my "feeling", I also found opposite arguments
> on the AMBER mailing list,
> like here: http://archive.ambermd.org/201103/0431.html
> So I'll got back again on my research and adjust my "mind" on the actual
> bleeding edge simulations
> taking into account all the recent code and force fields progresses.
> Best,
> Stéphane
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