[gmx-users] epsilon_rf

[Berk Hess]

>From: Volker Knecht <vknecht at mpikg.mpg.de>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: gmx users <gmx-users at gromacs.org>
>Subject: [gmx-users] epsilon_rf
>Date: Fri, 12 Jan 2007 12:04:34 +0100
>
>Dear GROMACS users,
>
>in the past I tended to use PME to calculate electrostatic interactions, 
>but to be able to reach larger timescales I would like to use reaction 
>field now whenever appropriate. I am wondering about an appropriate value 
>for the dielectric constant for a reaction field correction for simulations 
>of peptides in bulk SPC water or an SPC/bilayer interface. Whereas some 
>(group of van Gunsteren) have used the self-consistent value for SPC water 
>in simulations of pure SPC water, 54, most people seem to use the 
>experimental value of the dielectric permittivity of water at room 
>temperature, 78 (leading to an inconstent model). Often, authors using 
>reaction field do not even give the value for epsilon_rf chosen. Anezo et 
>al. have shown that both epsilon_rf = 54 and 80 yield similar properties 
>for lipid bilayer systems, and (besides PME) reaction field was one of the 
>recommended options for calculating electrostatic interactions for bilayer 
>systems. Strictly spoken, reaction field is only appropriate for a bulk 
>system, but it is widely used for simulations also of bilayer systems for 
>practical reasons. For a bilayer system, in principle, it is even less 
>clear which value for epsilon_rf should be chosen, since molecules in the 
>interface see environments with both very high and very low (close to 1) 
>dielectric permittivity.
>
>I have the feeling that very different opinions on the usage of reaction 
>field and epsilon_rf for biomolecular simulations, ranging from rigorous to 
>pragmatic, coexist in the community, and it would be nice if some of you 
>could provide their opinion.

One can probably have long discussions one which value of epsilon_rf
one should use. But such discussions are irrelevant.

For epsilon_rf=54 the prefactor for the RF term is 0.486,
for epsilon_rf=80 it is 0.491. This difference is negligible,
especially compared to the effect that the choice of cut-off
distance will have.
If you want to avoid the discussion, use an epsilon of infinity
(prefactor 0.5), this also gives the least cut-off effects,
as the potential will be exactly zero at the cut-off.

Berk.

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