The ABINIT Users Mailing List ( CLOSED )

[Michel Côté <michel.cote@umontreal.ca>]

Title: Re: [abinit-forum] FW:Cohesive Energy

Perfect!

Michel


Le 30/08/06 09:15, « LEE Chin Chai » <cc-lee@imre.a-star.edu.sg> a écrit :

> Dear Michel,
>
> Thank for your guidance. Finally, I managed to get the cohesive energy that is close to the experimental value. Indeed, I need to specify the occupancy number in order to achieve the lowest ground state energy for Si which is -3.7682 Ha and the cohesive energy turn out to be -0.1698 Ha/atom or -4.622eV/atom.
>
> Cheers,
> Chinchai
>
> -----Original Message-----
> From: Michel Côté [mailto:michel.cote@umontreal.ca] 
> Sent: Wednesday, August 30, 2006 2:54 AM
> To: forum@abinit.org
> Subject: Re: [abinit-forum] FW:Cohesive Energy
>
> Dear ChinChai
>
>
> Le 29/08/06 05:43, « LEE Chin Chai » <cc-lee@imre.a-star.edu.sg> a écrit :
>
> > Dear all ABINIT users,
> >  
> > Currently, I am trying out to test out a few pseudopotential files (GGA.fhi
> > format) for Co, Si, O and Ni before doing some supercell relaxation. But I
> > have some problems in calculating their respective cohesive energy. I started
> > on Si first as there is a tutorial that guides me (new user) along to use
> > abinit. In order to calculate the cohesive energy for Si, I would have to
> > calculate ground state energy of a Si atom. Can some one please guide me on
> > how to set the parameter for the input file for the calculation of ground
> > state energy of an atom? Is it a must to state their occupancy in order to
> > obtain the correct the ground state energy? If so, please give me some points
> > on how state the occupancy for different elements. Is it possible to do such
> > calculation by setting nsppol to 2 only?
> >  
> > I am using Abinit version 4.6.5. When I tried using nsppol to 2, the ground
> > state energy for Si is about -3.738 Hartree while the total energy for the 2
> > atoms primitive unit cell for Si is about -7.8761Hatree. So the cohesive
> > energy (E_coh = E_tot – 2*E_Si_atom) that I obtained is about 0.4001 Hartree
> > and it is far too big. If I am not wrong, the ground state energy that I have
> > calculated is wrong. But where have I gone wrong, please kindly advise. Thank
> > you.
>
> Actually, your cohesive energy is not so bad, 0.4 Ha = 10.9 eV/2 atoms ->
> 5.45 eV/atom. The experimental value is 4.63 eV/atom (Kittel). I think that
> GGA tends to overestimate the cohesive energy, therefore your value seems
> ok.
>
> You just have to make sure that you get the lowest energy possible for the
> atom as suggested by PMA. There are two ways to compute the energy levels of
> atoms, either you assign the occupancies of the orbitals or your let the
> code do it. For carbon atom,  it will look like:
>
> # User assign occupancies
> #Definition of the atom types
> ntype 1   
> zatnum 6
> nsppol 2
> nband 4 4
> occopt 2
> occ 1.0 2*1.0 0.0 1.0 3*0.0
>
> # Metallic case
> ntype 1   
> zatnum 6
> nsppol 2
> nband 4
> occopt 7
> tsmear 0.0005
>  
> A last comment about the metallic case, you have to be careful at possible
> degeneracies. In a cubic cell, the pi orbitals will be degenerate and the
> code will spread the electrons over all the orbitals whereas the lowest
> energy is when each electron occupy only one orbital as in the assigned
> case. If you still want to use the metallic case, you can remove the
> possible degeneracies numerically by using a slightly orthorhombic cell.
>
> Michel


-- 
*************************************************************** 
  Michel Cote                          tel: +1 (514) 343-5628     
  Professeur agrégé                   fax: +1 (514) 343-2071               
  Département de physique 
  Université de Montréal 
  C.P. 6128, succ. Centre-ville 
  Montréal (Québec) H3C 3J7          Michel.Cote@umontreal.ca 
  Canada                   www.phys.umontreal.ca/~michel_cote 
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