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Alexei Popov
alexei.a.popov@gmail.com

at your geometry, the energies of double-filled HOMO and
single-occupied orbital are almost identical; and this is
the origin of the problems you have encountered. There
exist at least two different solutions of SCF equations
nearly this particular geometry.

You can try to use SOSCF instead of DIIS. Alternatively,
restrict orbital mixing and increase the value of fshift, e.g.:

 
 $scf 
  diis=.t. dirscf=.t. diis=.t. rstrct=.t. shift=.t. fshift=0.2 
 $end 

The latter will in general have negative impact on the
convergence rate. As to SOSCF, it will have some problems
at the first geometry, but should rapidly converge at the
subsequent ones.

You may need to use multi-configuration methods to describe this system well.

regards,
Alexei

On Tue Sep 28 '10 10:25pm, Pedro Silva wrote
--------------------------------------------
>Symmetry is C1..
>The input follows as attachment. I have tried changing convergers to SOSCF , disabling DIIS, changing ICUT/ITOL, FDIFF=.f. or FDIFF=.t. (always without success)