Subject: CCL:Scan=Opt in G94 (fwd)

From: [email-masked] (Michael Frisch)
Date: Sat, 6 May 1995 16:53:59 -0400 (EDT)

Gavin Tasai asked about relaxed potential surface scans in Gaussian
94.  The defaults are enough different in G94 for both optimizations
and scans that this is worth a general reply.

In Gaussian 94 the default is to do optimizations, including relaxed
scans, in redundant internal coordinates.  This is done indentically
regardless of whether the initial structure is specified as cartesians
or a Z-matrix, and there is no connection between how the z-matrix, if
any, is specified and the coordinates used in the calculation.  When
information about the coordinates used in the optimization is
required, as is the case if you specificy Opt=AddRedundant to
add/change coordinates, Opt Geom=Modify to read in a structure but
then modify coordinates, or Scan=Opt (which is really synonymous with
Opt=AddRedundant since optimizations and relaxed scans are handled
identically), then the information is read in a format which allows
any of the possible redundant internal coordinates to be specified.
For example:

1 2 3 f 95.0

adds the angle between atoms 1, 2, and 3 if it is not already in
the redunant coordinate set and freeze it at 95 degrees.  Similary,

2 4 s 3 0.1

Makes the bond between atoms 2 and 4 a variable to be scanned with
3 steps of 0.1 angstroms.  In redundant optimizations, information
on z-matrix variable definition cards other than the value of the
variable has no effect.

It is also possible to do relaxed scans in z-matrix coordinates by
specifying Opt=Z-matrix on the route card and then indicating which
variables are to be scanned on the variable definition lines as is
done for non-relaxed scans.  Scan=(Opt,Z-Matrix) should be accepted as
a synonym for Opt=Z-Matrix, but it isn't.

Mike Frisch