Subject: Delocalized internal coordinates

From: [email-masked] (Folke Noertemann)
Date: Mon, 20 Apr 1998 16:44:42 +0200

Dear CCl'ers,
 my question is concerned with the generation of J. Baker's
'Delocalized Internal Coordinates' (J.Baker, A.Kessi and B.Delley,
J.Chem.Phys. 105 (1996)).
I have implemented them into my optimization code providing also
the possibility to generate them from Z-Matrix coordinates as
underlying primitive internals. Although this does not make
sense from the point of view of performance -delocalized internals
are only a linear transformation of Z-Matrix internals and
thus the optimization steps are identical- it is practical
to compare performance using different initial Hessians. 

As far as I understand, dihedral angles of 180 MUST be avoided
when using delocalized coordinates. In some cases, however, I find it
extremely difficult to generate a well-defined Z-Matrix without
these angles. The authors in the above reference mention that they
use an automatic generation of dummy atoms to handle this
situation. Unfortunately I simply cannot find out how to
place dummy atoms without ending up with a linearly dependent
set of vectors U (which make up the transformation matrix from
primitive to delocalized internals).
Did anybody else encounter this problem?

Thank you very much in advance!

Folke Noertemann
Lehrstuhl fuer Theoret. Chemie
TU Muenchen

[email-masked]