[gmx-users] Force constant defined in flexible TIP3P water model

David van der Spoel spoel at xray.bmc.uu.se
Sun Oct 5 14:15:19 CEST 2008

Bert wrote:
> Dear Gromacs Users,
> I have a simple question about the force constants of flexible TIP3P 
> water model defined in Gromacs. From the literature (THE JOURNAL OF 
> CHEMICAL PHYSICS 124, 024503 2006), the force constant for bond and 
> angle is 1059.162 kcal/mol/angstrom^2 and 68.087 kcal/mol/rad^2, which 
> is equivalent to 443153 kJ/mol/nm^2 and 285 kJ/mol/rad^2, 
> respectively. These two values are inconsistent with the corresponding 
> values in tip3p.itp, which gives 502416.0 kJ/mol/nm^2 for bond 
> and  628.02 kJ/mol/rad^2 for angle. I wanna know which one will be ok to 
> use. Thanks for your attention.

The original TIP3P model (JCP 79 (1983) 926) was rigid as far as I know. 
If you want a simple flexible model I would recommed TIP4P/Flex (Chem 
Phys Lett 372 (2003) 842). A literature search will probably reveal more 

> Bert
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David van der Spoel, Ph.D., Professor of Biology
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se

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