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

Alexandre Suman de Araujo asaraujo at if.sc.usp.br
Mon Jan 27 16:15:01 CET 2003

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
> _______________________________________________
> gmx-users mailing list
> gmx-users at gromacs.org
> http://www.gromacs.org/mailman/listinfo/gmx-users
> Please don't post (un)subscribe requests to the list. Use the www 
> interface or send it to gmx-users-request at gromacs.org.
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???

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

More information about the gromacs.org_gmx-users mailing list