[molpro-user] FCI iteration termination: expansion set singular

Tatiana Korona tania at tiger.chem.uw.edu.pl
Wed Jan 11 11:58:50 GMT 2012

Dear Steven,

If the basis set is large and your distance is small, this can be a numerical 
linear dependency. You can try tighter thresholds for integrals etc. 
(gthresh,twoint=1.e-25) and see if this helps.

Best wishes,


On Tue, 10 Jan 2012, Steven Fiedler wrote:

> Dear Molpro users:
> I am conducting a series of FCI calculations of multiple states of the He2* 
> dimer at various internuclear separation distances with a large basis set. 
> Calculations of small separation distances (< 2 bohr) terminate after a 
> number of FCI iterations with the following error: "expansion set singular". 
> Prior to this error, the lowest root appears to be converging rapidly to 
> minus infinity.
> Insight or advisement would be appreciated.
> Steven Fiedler
> -- 
> Department of Chemistry and Biochemistry
> California State University, Northridge
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Dr. Tatiana Korona http://tiger.chem.uw.edu.pl/staff/tania/index.html
Quantum Chemistry Laboratory
University of Warsaw
Pasteura 1, PL-02-093 Warsaw, POLAND

`The man who makes no mistakes does not usually make anything.'
                                        Edward John Phelps (1822-1900)

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