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

[Valentin Gogonea]

Hi Alexandre,

Sure, unfortunately I did not get anything up to now :-(

I will summarize if I will get something.

Valentin

On Monday, January 27, 2003, at 10:15 AM, Alexandre Suman de Araujo 
wrote:

> 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
>
>
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