[gmx-users] Proper Dih. multiplicity

David van der Spoel spoel at xray.bmc.uu.se
Fri Mar 8 19:54:28 CET 2002

On Fri, 8 Mar 2002, jozef hritz wrote:

>Sometimes, in the cases when I create my own .top file
>I get from grompp error: 
>Proper Dih. multiplicity can not be perturbed 2.000000!=6.000000

Point 1: for free energy purposes all interactions have two sets of
parameters A&B.

This is an inconsistency in your topology files. I fyou define a dihderal
with multiplicty 2 for the A type interaction (means two minima in the
cosine function, see manual) and the library has defined 6, than you have
to define 2 also for the B type interaction (since one can not
continuously change the periodicity of the cosine from 2 to 6 minima). 
This is a bit of a quirk in grompp, because when you do not use free
energy perturbation it shouldn't matter. However, whenever you do want
free energy perturbation, every definition that is different from the
library one contributes to the free energy.

Groeten, David.
Dr. David van der Spoel, 	Biomedical center, Dept. of Biochemistry
Husargatan 3, Box 576,  	75123 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://zorn.bmc.uu.se/~spoel

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