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.
Javier
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
--
Javier CEREZO BASTIDA
PhD Student
Physical Chemistry
Universidad de Mua
Murcia (Spain)
Tlf.(+34)868887434
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