[gmx-users] relative weight between the direct space sum and reciprocal space sum

[Mark Abraham]

Argyrios Karatrantos wrote:
> HI,
> 
> according to the 3.2 gromacs manual , the ewald_rtol parameter is the 
> relative strength of the electrostatic interaction at the cut off, 
> decreasing this gives a more accurate direct sum, but a less accurate 
> reciprocal sum(p. 81).
> The beta parameter is the parameter that determines the relative weight 
> between the direct sum and reciprocal sum(p.55) but i don't think you 
> can set the beta parameter in gromacs

As I said yesterday, you can choose its value, but you do so indirectly 
using rcoulomb and ewald_rtol. ewald_rtol controls the magnitude of the 
error in the direct space summation according to

ewald_rtol >= erfc( beta * r_{i,j,n}) / r_{i,j,n}

for all r_{i,j,n} > r_coulomb

Here, beta is a free parameter, and is chosen to satisfy the equality of 
the first equation. This value of beta is then used in the remaining 
Ewald terms.

Further, gromacs reports the value of beta being used in the .log file 
(search for beta)

Mark