[gmx-users] ewald_rtol parameter question

[Mark Abraham]

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.

No.

> 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.

Mark