[gmx-users] Question about Ryckaert-Bellemans function

David van der Spoel spoel at xray.bmc.uu.se
Tue Sep 18 20:23:01 CEST 2001

On Tue, 18 Sep 2001, Andrea Hombrecher wrote:

>For modelling liquid n-Octanol I started using the new version of the
>GROMOS96 force field ffG43a2x with the
>new LCH2 and LCH3 united atom types for hydrocarbons along with the
>Ryckaert-Bellemans dihedral potential.
>With the new parameters I can reproduce the density and the heat of
>vaporization very well, but the results for the
>structural properties (average transfraction) still remain quite bad.
>Now I would like to know, if it is possible to scale or to change the
>Ryckaert-Bellemans dihedral potential in
>order to improve the average transfraction.
I think that should be possible. Check first your current trans fraction,
and from that you can compute an effective free energy difference between
trans and gauche. Now you update the parameters in the potential (see
manual) to change the ratio. If you have too low trans fraction you must
make the gauche energy lower with respect to the trans-

Obvisouly you can also use OPLS or AMber parameters. I think these have
parametrized octanol too.

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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