[gmx-users] Bond force constant for GROMOS

Javier Cerezo jcb1 at um.es
Wed Jul 6 09:40:30 CEST 2011

Hi Felix

Take a look to 4.2.1 section in the manual. Gromos force constants (k^b) 
has this extra nm-2 factor because they are defined as:
2 k^b bij^2 = k^{b,harm}  (equation 4.39 in the manual version 4.5.4)

So you have to divide your derived harmonic force constants (in 
KJ/(mol*nm^2) by 2 and bij^2 (equilibrium distance or ideal bond length 
as stated in gromos files) to get the gromos one (k^b). It could also be 
useful for you to visit the new GROMOS website as there is some free 
downloadable FF documentation.


El 06/07/11 08:48, Rausch, Felix escribió:
> Dear Gromacs-users,
> I would like to add a new bond type to the ffbonded.itp of the 
> Gromos53a6 force field. For that, I tried to obtain the force constant 
> for my new bond by performing vibrational frequency calculations with 
> Gaussian. With that, I obtained the force constant in mDyne/A.
> My problem is now: How can I convert this value to kJ/(mol*nm(4)) as 
> required for the force field? I already tried, but I only end up at 
> kJ/(mol*nm(2)). Also arround the web I could only find a factor for 
> conversion of mDyne/A to kJ/(mol*nm(2)). Is there any trick or hint 
> you can give me about that?
> Thanks for your help,
> Felix

PhD Student
Physical Chemistry
Universidad de Mua
Murcia (Spain)

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