Q:choosing "zero freqs" to be projected out

["Stefan Fau" <fau()at()chemie.uni-marburg.de>]

 Dear CCLers,
 Gaussian (like many other programs) projects out the 6 "zero freqs"
 that belong to translation and rotation of a non-linear molecule before
 diagonalizing the remaining matrix. These "zero freqs" can be of
 similar magnitude as the lowest vibrational frequencies if the molecule is very
 floppy (I once had a molecule with 7 freqs of |ny|<15cm^-1).
 Does anybody know how the "zero freqs" are selected? Just by the
 absolute value of the raw frequency or is there some check for the type of
 movement (e.g. all atomic vectors of the mode parallel/orthogonal to the
 molecular translation/rotation vector) to avoid projecting out a vibrational
 frequency if vibrational and "zero" freqs are intermingled?
 I'll summarize if there is demand.
         Stefan
 ___________________________________
 Dr. Stefan Fau
 Fachbereich Chemie, AK Frenking
 Philipps-Universität Marburg
 35032 Marburg, Germany
 fau()at()chemie.uni-marburg.de