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Re: [ESPResSo-users] Bjerrum Length for Explicit Solvent

From: Ulf Schiller
Subject: Re: [ESPResSo-users] Bjerrum Length for Explicit Solvent
Date: Sun, 6 Oct 2013 13:37:30 +0200
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On 10/06/2013 11:22 AM, Peter Košovan wrote:
Similar trouble arises when we use point charges but replace the
continuum solvent with an explicit one. We can measure the dielectric
constant epsilon_model of our solvent model. This will probably differ
from the experimental value epsilon_exp since the model is not exact.

To my understanding, the solution is to adjust our bjerrum parameter
such that
1/epsilon_exp = 1/(epsilon_model * epsilon_bjerrum) where
epsilon_bjerrum is obtained by substituting the value of bjerrum
parameter we provided to Espresso into the common formula for Bjerrum

To this end, I do not see inconsistency between what Stefan and Marcus
have written.

Btw: does anyone know how users of other software deal with the problem
that epsilon_model does not match epsilon_exp? Do they use corrections
to epsilon similar to what I described or simply ignore the difference?

Hi Peter,

I think this is a software-independent modeling question. The Bjerrum length would be just one of potentially many input parameters of an explicit water model. If you take a given model and adjust only the Bjerrum length to match epsilon, it is not unlikely that you 'destroy' some other property like the diffusivity. Basically, the whole set of input parameters among with the degrees of freedom of the model have to be tuned to get the right physics. If I remember correctly, some SPC water models are able to reproduce the dielectric constant.


Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum Jülich, Germany          Fax:     +49 2461 61-3180

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