CCL:G: Gaussian: enforce symmetry

Dear Herbert,
Even though you would think a proper z-matrix description would do it, I think you should also specify opt=z-matrix to prevent optimisation within Gaussian's standard coordinates - redundant internals which can break this symmetry.
Also, you could enforce G03 to keep the input symmetry during optimisation by setting IOp(2/16=2):
http://www.ehu.es/sgi/ARCHIVOS/g03/g_tech/overlay_2.htm#2_16
HTH,
Fedor

On 13 October 2011 18:54, Herbert Fruchtl herbert.fruchtl() st-andrews.ac.uk <owner-chemistry^ccl.net> wrote:

Sent to CCL by: Herbert Fruchtl [herbert.fruchtl[a]st-andrews.ac.uk]
How do I prevent a Gaussian DFT calculation from changing symmetry during a geometry optimisation?

I have a largish molecule that's not flat in the gas phase, but I would like to optimise it like this anyway, which would presumably give me a higher-order saddle point. I start completely flat (all z-coordinates 0). The output reports Cs symmetry. But after the first geometry step, I get:

Omega: Change in point group or standard orientation.

...and from then on it's C1.

The grid I'm using is ultrafine. I also tried to enforce planarity using ModDredundant (basically freezing all dihedrals at 0 or 180 degrees), but then the iterative enforcement of ModRedundant doesn't converge (which tends to happen if there are too many constraints).

Is there a way to prevent this behaviour?

Thanks in advance,

 Herbert
--
Herbert Fruchtl
Senior Scientific Computing Officer
School of Chemistry, School of Mathematics and Statistics
University of St Andrews
--
The University of St Andrews is a charity registered in Scotland:
No SC013532



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