[gmx-users] ewald_rtol parameter question
gmx3 at hotmail.com
Wed Feb 18 16:29:54 CET 2009
> Date: Thu, 19 Feb 2009 01:53:47 +1100
> From: Mark.Abraham at anu.edu.au
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] ewald_rtol parameter question
> Claus Valka wrote:
> > Hello,
> > I would like to ask you a (simple) clarification question:
> > Reading the manual and equations 4.158 up to 4.160 there is a
> > convergence parameter b (with hellenic letters β).
> > I would like to ask you whether this parameter is the same as the
> > ewald_rtol that I come accross in the manual.
> > If it is not, is there any
> > function between those two quantities?
> Yes. It's something like erfc(R)/R <= ewald_rtol for R = beta *
> rcoulomb. That is, beta is controlled by the value of the two parameters
> so that the contributions to the direct-space sum from charges outside
> rcoulomb are negligibly small. Check the Darden papers referenced from
> the GROMACS manual for details. GROMACS reports the value of 1/beta in
> the .log file, IIRC.
ewald_rtol sets the relative magnitude of the direct space eletrostatics part
at the cut-off (as described briefly in the mdp options manual pages), i.e.:
erfc(rcoulomb)/rcoulomb = ewald_rtol
We should add a few lines on this to the manual.
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