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

[Ulf Schiller]

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
> length.
>
> 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.

Best,
Ulf

--
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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