[Pw_forum] How to obtain the binding energy ( in the unit of eV/Zn-O pair ) for Wurtsite ZnO?
zhaohscas at yahoo.com.cn
Mon May 30 13:49:54 CEST 2011
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:
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
> So to calculate the binding energy you have to use the below
> formula :
> 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
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?
> 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.
> 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
> 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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