# CCL: Spin contamination increased by annihilation of the first-order
contaminant

*From*: Mariusz Radon <mariusz.radon(_)gmail.com>
*Subject*: CCL: Spin contamination increased by annihilation of the
first-order contaminant
*Date*: Fri, 01 May 2015 23:45:15 +0200

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