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"