[angura]

Hi there,

sometimes there are certain atom combinations, that the usual force fields don't have parameters for.
I am in such a situation and was wondering, if a QM-software like Gaussian could help to determine the missing parameters.
This seems to be a quite offen used approach, but somehow I couldn't find a decent tutorial or explanation. (Please feel free
to post a link, if you know a good site)

What I have found out up to now:
- A "Freq" job on an optimized geometry at the same level of theory should be able to deliver force constants associated to certain normal modes

What I don't know:
- How do I relate these force constants to the bond- oder angle-parameters that I need? Just look at the normal modes in GaussView and guess?^^
- These force constants seem to be somehow unusually mass-weighed (http://www.gaussian.com/g_whitepap/vib.htm), so I guess I have to correct that somehow?

So basically the question is: How do I extract the force constants I would need for a MD simulation from such a Gaussian Freq Job?
Thanks in advance,
angu 



PS:
Here - as an example - the relevant output for a quick water simulation.
Maybe someone could explain it, based on this example.
Full mass-weighted force constant matrix:
 Low frequencies ---   -0.8180   -0.7763   -0.7607   -0.0012    0.0013    0.0015
 Low frequencies --- 1796.6481 4071.0854 4190.2192
 Diagonal vibrational polarizability:
        0.0000000       0.1336672       1.0326006
 Diagonal vibrational hyperpolarizability:
        0.0000000       0.0000000      -5.6012336
 Harmonic frequencies (cm**-1), IR intensities (KM/Mole), Raman scattering
 activities (A**4/AMU), depolarization ratios for plane and unpolarized
 incident light, reduced masses (AMU), force constants (mDyne/A),
 and normal coordinates:
                     1                      2                      3
                    A1                     A1                     B2
 Frequencies --  1796.6481              4071.0854              4190.2192
 Red. masses --     1.0828                 1.0451                 1.0839
 Frc consts  --     2.0592                10.2052                11.2123
 IR Inten    --   119.3046                23.3809                87.2098
 Raman Activ --     3.0418                74.8047                38.7365
 Depolar (P) --     0.6491                 0.1271                 0.7500
 Depolar (U) --     0.7873                 0.2256                 0.8571
 Atom AN      X      Y      Z        X      Y      Z        X      Y      Z
   1   8     0.00   0.00   0.07     0.00   0.00   0.05     0.00   0.07   0.00
   2   1     0.00  -0.43  -0.56     0.00   0.58  -0.40     0.00  -0.57   0.42
   3   1     0.00   0.43  -0.56     0.00  -0.58  -0.40     0.00  -0.57  -0.42