The force constants are the ones appropriate to each vibrational normal mode (VNM) obtained by your calculation. A VNM is generally delocalized (involves motion of multiple atoms simultaneously). One must fit a force constant model of some kind to the calculated VNM frequencies and masses in order to obtain force constants. The current Wikipedia page on molecular vibrations is helpful: http://en.wikipedia.org/wiki/Molecular_vibration . Unfortunately there probably isn't an automated way to obtain force constants, because one has to assume a particular kind of force constant model that is appropriate for the molecule at hand. There might be codes available that will take a Gaussian PES scan and produce the constants for a particular type of valence force field, but I can't think of any offhand.

There is one exception, though. Sometimes a particular VNM happens to be highly localized and can be described as an "X-Y stretch". In that case, the force constant of that VNM is approximately equal to the X-Y stretching constant. Hope this is helpful to you...

Dear Najme Dehghan,

I suppose this result are the normal modes of the molecule. Probably the vibrations you want to study are not presented in pure form.

In my opinion, you should do a scan of the bonds and angles, it will provide a potential energy function. Derive this function to get the force at each point and finally associate with Hooke's Law.

Dear Ranylson Marcello Savedra

How could i do a scan of the bonds and angles? if i understood your suggestion exactly, i should use Gauss-view and achieve bond length or angle value ,then use the related equation to calculate the force constant. am i wright? if yes,could you pleas give me some help with an example?

The force constants are the ones appropriate to each vibrational normal mode (VNM) obtained by your calculation. A VNM is generally delocalized (involves motion of multiple atoms simultaneously). One must fit a force constant model of some kind to the calculated VNM frequencies and masses in order to obtain force constants. The current Wikipedia page on molecular vibrations is helpful: http://en.wikipedia.org/wiki/Molecular_vibration . Unfortunately there probably isn't an automated way to obtain force constants, because one has to assume a particular kind of force constant model that is appropriate for the molecule at hand. There might be codes available that will take a Gaussian PES scan and produce the constants for a particular type of valence force field, but I can't think of any offhand.

There is one exception, though. Sometimes a particular VNM happens to be highly localized and can be described as an "X-Y stretch". In that case, the force constant of that VNM is approximately equal to the X-Y stretching constant. Hope this is helpful to you...

Exactly Najme Dehghan. See examples 3 and 4 of the attached site. It will help to run scans using the GAUSSIAN.

http://www.cup.uni-muenchen.de/ch/compchem/geom/scan.html

Robert Q. Topper and Ranylson Marcello Savedra,

I'm thankful because of your help.it was really useful for me.

best regards.

http://compbio.chemistry.uq.edu.au/mediawiki/upload/2/2a/Seminario_Hess2FF.pdf

best

Interesting paper, Hamed. I also found this: http://dissertations.ub.rug.nl/FILES/faculties/science/2002/m.swart/c3.pdf

Look up the method by Nicolai Lehnert at UMich Ann Arbor chemistry. He used to have a software that could use a UB matrix (user defined) to extract out the force constants of specific normal models. I think his approach was called QCNCA or something like it.

In doing a bond scan you can either fit the potential to a simple quadratic potential or to the Morse potential. It shouldn't make a difference for small displacements. However, you may need to consider the anharmonic potential as well.

Best,

Peter