Sent to CCL by: "Andrea Ciccioli" [andrea.ciccioli_-_uniroma1.it]
Dear friends,
it is commonly asserted in textbooks and software manuals that the obvious test
to recognize minima in PES among stationary points is that the vibrational
frequencies have to be real.
However, I wonder if besides this criterium one should also check carefully the
values of the frequencies reported as "Low frequencies" in Gaussian
outputs (just before the list of Harmonic frequencies). These are the
"frequencies" actually corresponding to translations and rotations,
and they should be ideally equal to zero, and indeed in many cases they are very
low. However, it happens not seldom to me, e.g. for triatomic species containing
heavy elements such as transition metals, to obtain outputs where, although the
harmonic vibrational frequencies are all real (positive numbers in the Gaussian
output), ie the structure should be a minimum, nevertheless one or two "Low
Frequencies" are not that low. Furthermore, they are in general both
positive and negative. For example, low frequencies as high as +/- 10 to 40
cm-1 are obtained. Moreover, these values are apparently larger for analytic
second-derivative frequency calculations than for numerical calculations (I use
DF!
T methods).
As far as you know, these relatively high values of the "Low
Frequencies" can indicate that the calculated structure is not a true
minimum, in spite of having real harmonic frequencies ? What could be a
reasonable criterium to consider the "Low frequencies" small enough to
be sure that the stationary point is a true minimum ?
Has anyone some suggestions/indications to give me ?
Thanks to all, and season's greetings.
Andrea Ciccioli
University of Rome (ITALY)
Sapienza
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