[gmx-users] Very large fluctuations in dg/dl

[Berk Hess]

>From: Stéphane Téletchéa <steletch at jouy.inra.fr>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>Subject: Re: [gmx-users] Very large fluctuations in dg/dl
>Date: Wed, 09 May 2007 11:11:06 +0200
>
>Patrick Fuchs a écrit :
>>Hi Gromacs users,
>>I have a few questions related to solvation free energy calculation via
>>thermodynamic integration.
>
>>#ifdef FLEX_SPC
>>#include "flexspc.itp"
>>#else
>>#include "spc.itp"
>>#endif
>
>>coulombtype            = generalized-reaction-field
>>rcoulomb               = 1.4
>>rvdw                   = 1.4
>>epsilon_rf             = 54.0
>
>I cannot state for other parameters, but i'm surprised for the 
>(relatively?) low value for epsilon_rf used here.
>
>For instance in Geerke and van Gunsteren (ChempPhysChem 2006, 7, 674-678), 
>the value used for reaction-field (not generalized) is 61 for SPC water 
>(referring to Heinz, van Gunsteren & Hunenberger, J Chem Phys 2001, 115, 
>p1125-1136).
>
>Is this value a special case for 'generalized-reaction-field' ?
>
>Thanks in advance for clarifications.
>
>Stéphane

What matters is not epsilon_rf, but 1/epsilon_rf.
Anything above roughly 20 does not make a significant difference any more.
Using 54 or 61 or infinity will not give measurable differences
in any property of the system, except for the fluctuation of the system 
dipole.

Berk.

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