[Pw_forum] How to obtain the binding energy ( in the unit of eV/Zn-O pair ) for Wurtsite ZnO?

[Hongsheng Zhao]

On 05/28/2011 10:18 AM, mohnish pandey wrote:
>
>
> On Sun, May 29, 2011 at 4:33 AM, Hongsheng Zhao 
> <zhaohscas at yahoo.com.cn <mailto:zhaohscas at yahoo.com.cn>> wrote:
>
>     On 05/28/2011 02:44 AM, mohnish pandey wrote:
>
>         Dear Hongsheng,
>          All the four bonds in ZnO wurtzite are equivalent.
>
>     But the four bonds have different bond lengths: the bond length
>     parallel to c axis is different from that of the other two bonds.
>      Why can you say are equivalent?
>     They have different orientation but the length are same. You can
>     check it by calculating the bond length using crystal  convention.
>     Just give the coordinate in crystal-convention in Xcrysden and you
>     can measure it.You will get all the bonds as same.
>
The Xcrysden is not so intuitive for measuring the bond lengths.  I use 
materials studio's visualizer to do this test.  And find they have 
different  bond lengths. Please see the snapshot for this from here:

http://h1.ripway.com/zhaohs/ZnO-b4-bond-length.tif


I also give you the corresponding cif file which I used to measure the 
bond lengths, just for your information. To obtain the cif file, from 
here: http://h1.ripway.com/zhaohs/ZnO.cif 
<http://h1.ripway.com/zhaohs/ZnO.cif>

>
>         So to calculate the binding energy you have to use the below
>         formula :
>         E_{binding-ZnO}=E{calculated-ZnO-wurtzite}-E_{Zn_{crystal-total}}-E_{E(1/2*O2}}.
>          Remember that you have to do spin-polarised calculation for
>         O2 otherwise it will give wrong answer.
>
>     I've learned that this is due to O atom's open-shell
>     configuration. Than what is the initial spin should I use for O2?
>
>     Furthermore, for ZnO wurtzite, should I do spin-polarised
>     calculation or not?
>
> No for ZnO you dont have to. I said for O2 only. You can do both the 
> unpolarised and polarised calculation, and you will find the 
> difference in bond energy and bond length too.

This is just the things that I cann't figure them out completely.  Let 
me give more detail descriptions of my understanding and my confusion:

1- For most open-shell atoms, there are non-spherical density due to 
unfilled shells and a non-zero spin state, say, for O atom. In this case,
if we want to obtain the atom energy via DFT calculation by using the 
big box model,  a spin-polarised calculation should be used.

2- When the open-shell atoms combined together to form molecular, say, O 
to O2, then open-shell orbitals will form covalent bonds. In this case, 
does it mean that the open-shell atoms configurations will be changed 
into close-shelled configurations?  If so, I think we should not do a 
spin-polarised calculation for O2, just a non spin-polarised calculation 
should be OK.

3- For the Wurtsite ZnO, the Zn atom is a closed subshell atom, but the 
O atom is a  open subshell atom.  When they combine to form Wurtsite 
ZnO, how we know the Wurtsite ZnO molecule should be open-shell or 
close-shell?


4- In one word, when we calculate the total energy for a atom/molecule, 
what's the criteria that determine the spin-polarised calculation should 
be done or not?

Regards.

>
>
>         Binding energy definition is same as above. And cohesive
>         energy is the difference in energy between the energy of
>         crystal and corresponding species in isolated phases. Below
>         are the some references.
>
>     Do you mean, for the calculation of cohesive energy, I should use
>     the big box model, then put one Zn in the big box, and O atom in
>     another box to obtain the total energy of each isolated phases?
>     Yea you have to use box model as you correctly mentioned.
>
>         http://arxiv.org/ftp/cond-mat/papers/0610/0610002.pdf
>         http://prb.aps.org/pdf/PRB/v70/i5/e054104
>
>
>     Regards.
>     -- 
>
>     Hongsheng Zhao<zhaohscas at yahoo.com.cn <mailto:zhaohscas at yahoo.com.cn>>
>     School of Physics and Electrical Information Science,
>     Ningxia University, Yinchuan 750021, China
>
>
>
>
> -- 
> Regards,
> MOHNISH,
> -----------------------------------------------------------------
> Mohnish Pandey
> Y6927262,5th Year dual degree student,
> Department of Chemical Engineering,
> IIT KANPUR, UP, INDIA
> -----------------------------------------------------------------


-- 
Hongsheng Zhao<zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

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