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

[Tatiana Korona]

Dear Evgeniy,

I have investigated this case. It turns out that we have a rare case that the 
two lowest eigenvalues are complex (i.e. their real part is the lowest :-) ). 
Usually such situations appear for much higher states. Probably I have to add a 
special information and to stop the calculation in the subroutine controlling 
the Davidson convergence.

With the production version you can see this if you add extended eomprint 
options, like

eomprint,davidson=4,residual=4 ! maybe this is a bit too much of printing

and look at the string "and after reordering". Below there will be the 
eigenvalues with real and imaginary parts listed separately. You will see that 
from some moment you will have a small (plus/minus 0.1e-3) imaginary part.

The possible existence of complex eigenvalues is a side effect of the 
nonsymmetric similarity-transformed Hamiltonian exp(-T)Hexp(T), so it is the 
feature, not the error.

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