From chemistry-request&$at$&www.ccl.net Thu Feb 4 10:03:39 1999 From: "Stefan Fau" To: "CCL" Subject: RE: G:Freq in G94 Date: Thu, 4 Feb 1999 16:05:07 +0100 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"