[Pw_forum] [Fwd: paramagnetic structure]

[Davide Ceresoli]

Subhradip Ghosh wrote:
> Dear Davide,
> 
> Thanks for the reply.
> I guess I understand that a calculation can be only 'non-spin-pol' or
> 'spin-pol'(not PM,FM,AFM etc.) . But, don't we use phrases like these
> during discussions? Atleast, I have been in discussions where people say
> 'PM calcs or FM calcs' which nobody interprets in the literal sense!
Dear Subhradip,
     still I don't understand the difference between a "paramagnetic
calculation" or a "ferromagnetic calculation". Do you mean that in one
case the total spin magnetization is zero and in the other case it
is different from zero?
Take for instance the oxygen molecule. Would you perform a "paramagnetic"
calculation or a "ferromagnetic" one? and for bulk iron instead?

> Nevertheless, let me be more specific.
> How does espresso simulate a paramagnetic state  for a system having
> unpaired spins ( a system which does not have closed shells).
Take again the simplest case of the oxygen molecule. We all now that
the ground state is a triplet (a di-radical). So the calculation will
be spin-polarized:

   occupations = 'fixed'
   nspin = 2
   multiplicity = 3   (multiplicity = 2S+1)

Since there are 12 valence electrons, this will place 7 electrons in
the spin up channel and 5 electrons in the spin down channel.

Davide