[Pw_forum] Something strange in the calculation with the spin=2 and multiplicity=1

[Giuseppe Mattioli]

Dear Vega

> Do the both calculation output the singlet state energy?

They are two different states: 
'spin=2, multiplicity=1' --> Here you can deal with a system with two, 
spatially different, single-electron KS wavefunctions, occupied by one 'up' 
electron and one 'down' electron. This can be called an "open shell singlet" 
You could find that: a) your total magnetization is 0.00 mB, because this is 
forced by the  'multiplicity=1' keyword, but your absolute magnetization could 
be, say, 2.00 mB, a sort of antiferromagnetic system; b) the energy difference 
with a real triplet, in this case, could be almost negligible, so that the 
system could be considered a triplet forced in this state due to the 
'multiplicity=1' constraint
'spin=1' --> Here you are forcing a restricted representation of your system, 
that is, you have KS wavefunctions that can be occupied by a couple of paired 
electrons.
Which is the 'best' singlet? You should know, depending upon the properties of 
your system.
Hope this help

Giuseppe

On Wednesday 13 January 2010 11:11:14 vega lew wrote:
> Dear sir,
>
> Do the both calculation output the singlet state energy?
> If I want to compare the energy difference of singlet and triplet, which
> calculated singlet state energy should be used?
>
> thank you for your help.
>
> best wishes,
>
> vega
>
> On 1/13/2010 14:43, Paolo Giannozzi wrote:
> > On Jan 13, 2010, at 6:41 , vega lew wrote:
> >> I have compare the results of two calculation for singlet state
> >> with or without the option 'spin=2, multiplicity=1'.
> >
> > they aren't the same. A non-polarized calculation will yield S^2=0,
> > Sz=0.
> > A spin-polarized calculation with "multiplicity=1" will yield Sz=0
> > but not
> > necessarily S^2=0. See for instance J. Phys. Chem. B 105, 11227
> > (2001) for a nice description of how neglecting to appreciate such
> > difference may lead to unphysical results.
> >
> > P.
> > ---
> > Paolo Giannozzi, Dept of Physics, University of Udine
> > via delle Scienze 208, 33100 Udine, Italy
> > Phone +39-0432-558216, fax +39-0432-558222
> >
> >
> >
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum


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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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