[gmx-developers] ewald_rtol / beta

[Dimitris Dellis]

Hi gmx_developers.

In manual (v4.5) p. 100 ewald_rtol parameter is referred as  "The 
ewald_rtol parameter is the relative strength of the electrostatic 
interaction at the cut-off"
According to this (and some older posts in lists), beta parameter is 
obtained by solving equation
ewald_rtol = erfc( beta * r_c) / r_c     [1]
In gmxlib/ewald_utils.c, function calc_ewaldcoeff()  (versions 3.3.4 - 
4.5.4) beta is obtained by solving eq.
ewald_rtol = erfc( beta * r_c)             [2]
(division by r_c omitted ).
These 2 expressions give the same beta only if r_c=1.
I tried to investigate convergence of real/recip. sums as function of 
beta. For certain (beta, r_c), I calculate ewald_rtol using [1], put 
this ewald_rtol to mdp.
In md.log the reported beta is not the same with the one that was used 
to calculate ewald_rtol (not only last few digits deviation), except if 
r_c=1.
If I calculate ewald_rtol using [2] and give this value in mdp, mdrun 
reports the expected beta.
In almost all systems this doesn't affect results since real/recip. sums 
converge for values of beta slightly different (few %) than the supposed 
beta,
and typical rcoulomb is close to 1.
This might affect results in some cases that exhibit difficult (if not 
impossible) real/recip. sums convergence, that for various reasons 
require large cut-offs.

Is this a bug ?

DD





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