The ABINIT Users Mailing List ( CLOSED )

[Anglade Pierre-Matthieu <anglade@gmail.com>]

On Fri, Mar 6, 2009 at 5:49 PM, Souraya Goumri-Said
<Souraya.Goumri-Said@fundp.ac.be> wrote:
> 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

nband= ne/2 +2 (at least)


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

have a look at :
http://www.abinit.org/Infos_v5.5/input_variables/varbas.html#occopt
You will see that using occopt = {1, 3, 4, 5, 6, 7}
is easy while other options are used in very special cases.
For a semiconductor it is natural to use occopt 1 yet any of the
choice above will work as well.



>
> 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
>



-- 
Pierre-Matthieu Anglade