[gmx-developers] ewald_rtol / beta
Dimitris Dellis
ntelll at gmail.com
Mon Jun 6 23:55:46 CEST 2011
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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