[Pw_forum] fixing spin in LSD

[Eduardo Ariel Menendez P]

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?




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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/