[gmx-users] gain the force field parameter by quantum mechanics calculation

[David van der Spoel]

On Mon, 2005-04-18 at 10:37 +0800, zjim wrote:
> Many thanks, David
> I did the frequency calculation, and i got the bending force constant
> (0.2331) and the stretch force constant(0.8402). but I can't associate
> them to the force constant in the opls FF. and as a test,i also
> calculate the stretch force constant of H-C(2.4214) and C-C (6.1634),
> the value is not proportional to the value in opls ff (C-C:224412.5 C-
> H:284702.4). can you give some more advice, thank you again.
> 
How about units? Doesn't Gaussian give you hartree/bohr^2 or something?
I can imagine that the bond strength is quite a bit stronger in H-C than
in  I-I, especially since your molecule is charged.

> the frequency calculation output as follow
> 
> C-C
> ?A 
> Frequencies -- 1063.9177
> Red. masses -- 3.6307
> Frc consts -- 2.4214
> IR Inten -- 0.0000
> Raman Activ -- 16.1188
> Depolar -- 0.2891
> 
> C-H
> Frequencies -- 3182.0979
> Red. masses -- 1.0331
> Frc consts -- 6.1634
> IR Inten -- 72.1900
> Raman Activ -- 0.0000
> Depolar -- 0.0302
> 
> I-I-I(bending)
> 1 
> PIU 
> Frequencies -- 55.8400 
> Red. masses -- 126.9004 
> Frc consts -- 0.2331 
> IR Inten -- 0.0874 
> 
> I-I-I(stretch )
> 3 
> Frequencies -- 106.0065
> Red. masses -- 126.9004
> Frc consts -- 0.8402 
> IR Inten -- 0.0000
> 
> 
> 
> 
> 
> > On Sat, 2005-04-16 at 11:11 +0800, zjim wrote:
> 
> > > hi every one
> 
> > > when there is no parameter for some atom types of a molecule, such
> as
> 
> > > triiodide ion . Quantum mechanics calculation method (DFT MP2)
> seems
> 
> > > to a reasonable solution. the partial charge is easy to calculate
> by
> 
> > > add the key words "ChelpG", but how to calculate the force
> constant
> 
> > > (fc) of the bond , angle ,together with dihedral. If there is
> someone
> 
> > > know, please give some advice. 
> 
> > > thank you very much!
> 
> > In principle you can deduce it from a frequency calculation.
> 
> > 
> 
> > > 
> 
> > > Zhou Jin ming
> 
> > > Shanghai Institute of Organic Chemistry
> 
> > > Computational Chemistry Lab
> 
> > > 
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> > > 
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> > > 什么邮箱可以设置 图片签名档，发送电子名片？ 
> 
> > > 
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> > > 
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> > > 什么邮箱可以设置 图片签名档，发送电子名片？ 
> 
> > > 
> 
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> > -- 
> 
> > David.
> 
> >
> ________________________________________________________________________
> 
> > David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
> 
> > Dept. of Cell and Molecular Biology, Uppsala University.
> 
> > Husargatan 3, Box 596, 75124 Uppsala, Sweden
> 
> > phone: 46 18 471 4205 fax: 46 18 511 755
> 
> > spoel at xray.bmc.uu.se spoel at gromacs.org http://xray.bmc.uu.se/~spoel
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-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,          75124 Uppsala, Sweden
phone:  46 18 471 4205          fax: 46 18 511 755
spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/~spoel
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