[Pw_forum] fixing spin in LSD

[Stefano de Gironcoli]

For a magnetic system the code looks for a self-consistent solution.
In many cases this solution is easily found, in some other cases it may 
be difficult.
This may be due to the fact that different solutions (with different 
magnetizations) are possible and the code is not able to choose. A 
possible strategy then is to constrain the magnetization to several 
values and calculate the scf solution for each. This allows to 
determine, a posteriori, the GS of the system and also have some idea of 
the low-lying alternatives that may be of some physical interest.
Constraining the magnetization is one of the several tools we have in 
order to ask questions to the system.... answers will be meaningful, if 
the questions are.

Stefano de Gironcoli, SISSA and DEMOCRITOS

Eduardo Ariel Menendez P wrote:
> Hi,
>
> My doubt may be trivial,as I cannot find the answer anywhere.
>
> When does one needs to fix the value of the spin in LSDA calculation?
>
> multiplicity   INTEGER ( default = 0 [unspecified] )               spin
> multiplicity (2s+1). 1 is singlet, 2 for doublet etc.               Note
> that this fixes the final value of the magnetization.               if
> unspecified or a non-zero value is specified in nelup/neldw
> then multiplicity variable is ignored.
>
> In particular, I am interested in the case of impurities in surfaces.
> Should I fix the spin in this case or not?
>
> Other case is dimers and clusters. I have answered this question to
>  senior quantum chemists  and I have received answers like
>
> a) Whith Hartree-Fock equations you must specify the spin, and DFT works
> well when HF does, and fails when HF fails.
>
> b) The states measured by spectroscopy have well defined S_z and S^2.  So
> you want to calculate these states.
>
> Is there any other (and more fundamental) reason?
>
>
> Polite request:
> Avoid send Word attachments. To see why, take 6 minutes to read at
> http://www.gnu.org/philosophy/no-word-attachments.html
>
> Eduardo A. Menendez Proupin
> Department of Physics
> Faculty of Science
> University of Chile
> Las Palmeras 3425
> Ñuñoa, Santiago
> Chile
> Phone: (56)(2) 978 74 11
> Fax  : (56)(2) 271 29 73
> http://fisica.ciencias.uchile.cl/~emenendez/
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