From owner-chemistry@ccl.net Fri May  1 17:47:00 2015
From: "Mariusz Radon mariusz.radon::gmail.com" <owner-chemistry{:}server.ccl.net>
To: CCL
Subject: CCL: Spin contamination increased by annihilation of the first-order contaminant
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X-Original-From: Mariusz Radon <mariusz.radon(_)gmail.com>
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Date: Fri, 01 May 2015 23:45:15 +0200
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Sent to CCL by: Mariusz Radon [mariusz.radon]_[gmail.com]
On 05/01/2015 05:00 PM, Igors Mihailovs igors.mihailovs0::gmail.com wrote:
> Dear all,
> 
> the spin
> contamination was not descreased by removal of the first-order contaminant:
>  (...)
>  Annihilation of the first spin contaminant:
>  S**2 before annihilation     2.1080,   after     8.9661
>  
> 
> Is there some way to interpret this result?
> 

Dear Igors:

Yes, this could happen if your wave function is contaminated by higher
spin states (not only S+1, but also S+2, S+3, etc; where S = M_S is the
desired spin state; zero in your case).


Take a look at the following paper: Davidson, E. R. & Clark, A. E. "Spin
Polarization and Annihilation for Radicals and Diradicals"
Int. J. Quantum Chem., 2005, 103, 1-9. The most relevant conclusion:
"The annihilation procedure greatly increases the weights on the higher
spin states and can easily lead to an increase, rather than a decrease,
in <S^2>."

You have probably a biradical or multiradical system. In such case,
analysis of natural orbitals (eigenvectors of the density matrix) or
natural spin density orbitals (eigenvectors of the spin density matrix),
or Amos-Hall corresponding orbitals can be useful to identify the pairs
of weakly coupled electrons in your system.

Best regards,
Mariusz


-- 
Dr Mariusz Radon, Ph.D.
Coordination Chemistry Group
Faculty of Chemistry
Jagiellonian University
ul. Ingardena 3, 30-060 Krakow, Poland
http://www2.chemia.uj.edu.pl/~mradon