[molpro-user] Simple unreasonable norm problem...

Gerald Knizia knizia at theochem.uni-stuttgart.de
Fri Oct 15 11:54:00 BST 2010

On Thursday 14 October 2010 19:38, Charles Wood wrote:
> My problem is this:
> I am trying to run an extremely simple H - Ca2+ - H calculation (one
> calcium 2+ ion centred between 2 hydrogen atoms). I'm using UHF and
> UCCSD(T) programs in molpro2009.1. If the geometry is specified so each
> of the H atoms are less than 1.5 Angstroms then the calculation works fine.

At some bond stretching far beyond the equilibrium geometry, the molecule will 
become multi-referency (due to the near-degeneracy of the molecular state and 
the dissociated states). So starting at some geometry CCSD(T) will give bogus 

For your 1.5 Angstrom geometry this is already the case. If you run mcscf and 
ci, you will see that there are two reference configurations with comparable 

> I saw some people suggest 'shift' but this looks like something
> connected to the geometry....any idea??

If you want to see the CCSD(T) blowing up instead of giving this message, you 
can include
in the input. It will then happily converge without giving this message.
Gerald Knizia

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