# RE: G:Freq in G94

Dear Krys,
the vibrational frequencies are derived from the force-constant matrix(second
derivatives of the energy wrt. nuclear coordinates). In a non-linear molecule 6
of the 3*N_atoms degrees of freedom describe translation and rotation of the
molecule. If there were no numerical inaccuracies and if the geometry was
perfectly minimized, these 6 freqs would be zero since the SCF energy is
independent of the orientation and location of the molecule.
Because of the mentioned imperfections, three of the 6 "zero-freqs"
are close to zero and the other three "zero-freqs" have low absolute
values (typically in the range of 10 to 50 cm-1, depending on the quality of the
geometry optimization). To avoid coupling, these "zero-freqs" are
projected out and the remaining 3*N_atoms-6 x 3*N_atoms-6 matrix is
diagonalized.
For more details have a look into, e.g. "Introduction to Computational
Chemistry" by Frank Jensen. It gives a good overview and is really
affordable (~ DM 80).
Stefan
___________________________________
Dr. Stefan Fau
Fachbereich Chemie, AK Frenking
Philipps-Universität Marburg
35032 Marburg, Germany
fau |-at-| chemie.uni-marburg.de
> I have question concerning log from IR calculation in G94.
> Below is part of this log:
> > Low frequencies --- -82.8125 -5.2461 -0.0021 0.0021 0.0064
7.1252
> > Low frequencies --- 10.1830 65.1288 69.2516
> > ****** 1 imaginary frequencies (negative signs) ******
> > Harmonic frequencies (cm**-1), IR intensities (KM/Mole),
> > Raman scattering activities (A**4/AMU), Raman depolarization ratios,
> > reduced masses (AMU), force constants (mDyne/A) and normal
coordinates:
> > 1 2 3
> > A" A"
A'
> > Frequencies -- -82.7948 65.1108
69.2511
> > Red. masses -- 1.0165 1.0212
1.1460
>
> First frequency is 1A"= -82.7948, and I can also see it in first line
> that I gave:
> "Low frequencies --- -82.8125" The second is 65.1108 cm^-1, so
my
> question what
> are this "-5.2461 -0.0021 0.0021 0.0064 7.1252
10.1830"