[gmx-users] bond stretching force constant from QM data

[Alexandre Suman de Araujo]

Valentin Gogonea wrote:

> Dear CCLers and GMX users,
>
> I am trying to estimate the force constant for Fe-S bond stretching 
> from QM calculations and use it with Gromos87 force field (gromacs). I 
> carried out UB3LYP/6-31+G** calculations for Fe-SCH3 
> (charge=+1,multip=5) and Fe-Imidazole (+2,5) with G98 and I got the 
> following force constants (I used a conversion factor from mDyne/A to 
> KJ/(mol nm2) of 60230):
> mDyne/A kJ/(mol nm2) Gromos87
> Fe-S 1.839 110763
> Fe-N 0.757 45594 418400
>
>
> 1) Is it acceptable to scale properly (using the QM and Gromos Fe-N 
> force constants) the Fe-S force constant to be in the same range as 
> the Fe-N force constant in the Gromos87 force field. Did anybody use 
> this kind of approach or there is a better way to estimate and adjust 
> the QM determined force constant to a force field force constant?
>
> 2) For an angle bending the force constant required in the forcefield 
> has units of kJ/(mol rad2), while the one obtained from QM 
> calculations has units of kJ/(mol nm2). I believe that the former can 
> be obtained from the latter if one assumes that the oscillator length 
> for this particular normal mode is the distance between the endpoints 
> of the angle. This should be valid for a torsion angle too. Is this 
> correct?
>
> I would appreciate any insight that I can get from the list and I 
> thank you for your help.
>
> Valentin Gogonea
> Department of Chemistry
> Cleveland State University
>
>
>
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Hi Valentin

I have the same problem that you and I'm very interested in the answers 
that you can get. If you receive anyone in private, can you send it to me???
Thank's

-- 
Alexandre Suman de Araujo
asaraujo at if.sc.usp.br
UIN: 6194055
IFSC - USP - Sa~o Carlos - Brasil