Subject: G92 symmetry question

From: [email-masked] (Bao-Liang Tsai)
Date: 17 Aug 1995 08:58:38 GMT

How does Gaussian 92 recognize molecular symmetry?  I have a
molecular geometry with perfect C2v symmetry in Z-matrix, but
G92 just won't acknowledge it.  Is there any trick to solve
the problem?  Advices will be very much appreciated.

-Bao

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From: "Sergei Vyboishchikov" <[email-masked]>
Date: 17 Aug 95 12:47:16 MDT

Dear Bao-Liang,

As far as I know, the Gaussian
- takes the molecular geometry
- calculates the tensor of inertia
- diagonalizes it
- determines the principal axes of inertia and the type of top:
 -- asymmetrical, which can occur only in C1 and Ci point groups
 -- symmetrical, which can occur only in subgroups of D2d (e.g., in C2v)
 -- spherical, which can occur only in high symmetric groups (e.g., in Oh or
                                                                   Td)
- transforms the geometry to the coordinate system of axes of inertia
   (so-called "standard orientation")
- finally, determines the point group

Note that all this actions are done with the actual molecular geometry
regardless of the Z-matrix. Normally, Gaussian does it perfectly.
However, it can unfortunately occur that the particular molecule is by
occasion a spherical top (i.e. all three components of traceless inertia
tensor are occasionally equal within the thresholds used by Gaussian) but
it does not belong to any high symmetric group. In this case, the
Gaussian gives a corresponding error message. If it is really the case,
one can try to change the atomic weights of some atoms to avoid this
difficulty. If you do not receive any error message, it means most
likely that your Z-matrix is wrong and the molecule has no C2v symmetry.

                              Regards,
                                      Sergei Vyboishchikov
                                      Universitaet Marburg