[gmx-users] reaction field kappa and ionic strength

Daniela S. Mueller d.s.mueller at rug.nl
Fri Mar 23 15:24:02 CET 2007

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?


best regards,


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

More information about the gromacs.org_gmx-users mailing list