[molpro-user] Unclear problem in EOM-CCSD calculations: "small eigenvalue of S-eff matrix ... This can lead to convergence problems!"

[Tatiana Korona]

Dear Evgeniy,

This message can signify near-linear dependencies in the small space in the 
Davidson procedure. One can try to a) increase the quality of the integrals (for 
the case when this dependence is caused by numerical noise of orbitals and CCSD 
amplitudes - note that if CCSD is not a final step, a bit more accurate 
T1+T2 amplitudes are often required); b) try to play with Davidson options, e.g. 
change MAXDAV.

Another issue is a high energy of the virtual orbital. I presume you want to 
obtain some high-lying states, what often means that there is a large density of 
states having a significant component of the excitation to this orbital and 
then the procedure will jump from one state to another and back (or to yet 
another state). In this case sometimes it helps if you ask for more states (but 
only sometimes). In general finding high states is a difficult no-black-box 
task, unfortunately :-(

Best wishes,

Tatiana

On Tue, 6 Jun 2017, Evgeniy Gromov wrote:

> Hello,
>
> I encountered some unclear problem when doing EOM-CCSD calculations
> for an Ir complex. All steps till EOM went fine, but in EOM I get the
> following warning:
>
> The smallest eigenvalue of S-eff matrix     0.8656157915E-13
> This can lead to convergence problems!
>
> and two of the five roots do not seem to have converged; there is
> no  "Converged" message for them after 50 iterations.
>
> I wonder what this could be caused by? Could it be due to a very high
> in energy (63.5 hartree) virtual orbital?
>
> Best regards,
> Evgeniy
>
> -- 
> Dr. Evgeniy Gromov
> Theoretische Chemie
> Physikalisch-Chemisches Institut
> Im Neuenheimer Feld 229
> D-69120 Heidelberg
> Germany
>
> Telefon: +49/(0)6221/545213
> Fax: +49/(0)6221/545221
> E-mail: evgeniy.gromov at pci.uni-heidelberg.de
>
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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