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[Souraya Goumri-Said <Souraya.Goumri-Said@fundp.ac.be>]

Dear PMA,

Thank's very much for you detailed explanation

I still have some confusion <:-| : 

Suppose we have a system of 8 atoms (A, B and C) , and we have 4 A atoms,
3 B atoms and 1 C atom

following your explanation: the valence electron : ne = 4 x ZION(A)+ 3 x
ZION(B)+ ZION(C)  ( ZION extracted from PSPNC files) ( is it correct
?)

which _expression_ is correct : nband = 1/2 [  ne+2 ] or  nband=
ne/2 +2 

occopt=1 is it the best choice or can be equal =< 3 for SC

Best regards,

Souraya,





At 09:18 6/03/2009, you wrote:
> For basic ground state
> calculation in semiconductors, nband is to be
> chosen has, at least, half the number of electrons (ne)  within
> the
> cell + 2. The number of electrons within the cell is the sum of all
> the valence electron of the atoms in the cell. The number of valence
> electrons of an atom is written in the pseudopotential file you use
> for this species. If you have a look at Abinit pseudopotentials,
> they
> contain a variable zion. This is the number of valence electron of
> the
> atom.
>
> A DFT calculation consist in writting an hamiltonian (made from your
> atomic positions, pseudopotential, cell size, cutoff energy,
> exchange
> correlation energy) and getting the eigenvalue (energy) associated
> with each band (at convergence the bands are eigenvectors of your
> hamiltonian). When solving the problem one seeks only the lowest
> eigenvalues and associated eigenvectors since electrons are going to
> occupate only the lowest energy levels. For semiconductors, the
> occupied bands (vlaence) are separated from the unoccupied one
> (conduction) by a gap. Therefore, all the valence bands are totally
> occupied and all the conduction bands are totally empty ; then the
> laters make no contribution to the total energy and it is not
> usueful
> from a physical point of view to calculate them. Nevertheless
> including a few conduction bands helps solving the hamiltonian and
> that's why I recommended you to use at least nband = ne/2 +2 for a
> semiconductor. That's a minimum. For safety, if you don't understand
> very well, you'd better use nband = ne/1.5.
>
> In metals things are a little more complicated. Since conduction
> bands
> and valence bands are very close, temperature can spread the
> electrons
> over the fermi level. Moreover getting an accurate evaluation of the
> total energy is more difficult and can be made easier by using a
> high
> temperature for the electrons. Depending on the way you want to deal
> with those two facts you can choose with occopt a distribution for
> your population of electrons whose parameter is the electronic
> temperature (tsmear). Yet, if you actually deal with semiconductors
> you can safely choose occopt 1.
>
> regards
>
> PMA
>
>
>
> On Thu, Mar 5, 2009 at 7:42 PM, lagoun brahim
> <lag17_brahim@yahoo.fr> wrote:
> > HI
> > i am not a devloper then if i am wrong someone  should correct
> me
> > we need a convergence study for ecution in the case of metals, but
> for
> > insulator was not  need that,
> > OCCUP x 2 =the number of valence electrons in your pseudopotentiel
> (in the
> > cell)
> >
> >
> > --- En date de : Jeu 5.3.09, Souraya Goumri-Said
> > <Souraya.Goumri-Said@fundp.ac.be> a écrit :
> >
> > De: Souraya Goumri-Said <Souraya.Goumri-Said@fundp.ac.be>
> > Objet: [abinit-forum] nband value and occopt
> > À: forum@abinit.org
> > Date: Jeudi 5 Mars 2009, 17h56
> >
> > Dear Abinitors,
> >
> > I have some problem to understand the "choice" of the
> nband and occopt value
> > ? By reading the input variables for nband and occopt, I don't know
> how can
> > I attribute the values of these input essentially for an
> insulator  (
> > semiconductors) ?
> >
> > Can someone explain to me how to know in adavance  these
> values ?
> >
> > Regards,
> >
> >
> >
> > Cordialement,
> > Sincerely yours,
> >
> > ====================================
> > Dr. Souraya Goumri-Said
> > Centre de recherche en Physique de la Matière et du Rayonnement
> (PMR)
> > Laboratoire de Physique du Solide
> > Facultés Universitaires Notre-Dame de la Paix
> > Rue de Bruxelles 61 - B - 5000 Namur
> > Tel : +32(0)81 72 47 01
> > Fax : +32(0)81 72 47 10
> >
>
> http://sourayagoumri-said.weebly.com/
> > email : Souraya.Goumri-Said@fundp.ac.be
> >
> >
>
>
>
> -- 
> Pierre-Matthieu Anglade

Cordialement,
Sincerely yours,

====================================
Dr. Souraya Goumri-Said
Centre de recherche en Physique de la Matière et du Rayonnement (PMR)
Laboratoire de Physique du Solide
Facultés Universitaires Notre-Dame de la Paix
Rue de Bruxelles 61 - B - 5000 Namur
Tel : +32(0)81 72 47 01
Fax : +32(0)81 72 47 10
http://sourayagoumri-said.weebly.com/
email : Souraya.Goumri-Said@fundp.ac.be