[gmx-users] reaction field kappa and ionic strength

[Daniela S. Mueller]

dear all,

there are errors in the code regarding 1) the equation for kappa, which 
is used for the reaction field method; and 2) in the calculation of the 
ionic strength.

1) in rf_util.c kappa is calculated as
*kappa  = sqrt(2*I/(EPSILON0*eps_rf*BOLTZ*Temp));

but the factor 2 in the square root is only correct if the sum in the 
ionic strength I is actually divided by 2, which it isn't. so the factor 
2 should be omitted.

2) in force.c the ionic strength is (among others) calculated by summing 
the absolute values instead of the squares of the charges.
zsq += fabs(q);
this is only correct for monovalent charge groups.

@developers: these errors have been remarked previously on the mailing 
list. as it is not mentioned in the bugzilla, i thought i can bring it 
up again... can you correct this in the code for the next version please?

thanks.

best regards,
daniela

-- 

Daniela S. Mueller

Biologist (Dipl. Biol.)

Molecular Dynamics (MD)
Groningen Biomolecular Sciences and Biotechnology Institute (GBB)

MD group website: http://www.rug.nl/gbb/md

Daniela Mueller
Dept. of Biophysical Chemistry
University of Groningen
Nijenborgh 4
9747 AG Groningen
The Netherlands

Tel.: +31 (0)50 3634327
Fax.: +31 (0)50 3634398
e-mail: d.s.mueller at rug.nl