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On 05/28/2011 10:18 AM, mohnish pandey wrote:
<blockquote
cite="mid:BANLkTimNgBvuJ0FVinuXOX+FEN8pmaGxWw@mail.gmail.com"
type="cite"><br>
<br>
<div class="gmail_quote">On Sun, May 29, 2011 at 4:33 AM,
Hongsheng Zhao <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:zhaohscas@yahoo.com.cn">zhaohscas@yahoo.com.cn</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div class="im">On 05/28/2011 02:44 AM, mohnish pandey wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> Dear Hongsheng,<br>
All the four bonds in ZnO wurtzite are equivalent.<br>
</blockquote>
</div>
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?
<div class="im">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.<br>
</div>
</blockquote>
</div>
</blockquote>
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: <br>
<br>
<a class="moz-txt-link-freetext" href="http://h1.ripway.com/zhaohs/ZnO-b4-bond-length.tif">http://h1.ripway.com/zhaohs/ZnO-b4-bond-length.tif</a><br>
<br>
<br>
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: <a href="http://h1.ripway.com/zhaohs/ZnO.cif">http://h1.ripway.com/zhaohs/ZnO.cif
</a> <br>
<br>
<blockquote
cite="mid:BANLkTimNgBvuJ0FVinuXOX+FEN8pmaGxWw@mail.gmail.com"
type="cite">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div class="im"> <br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> So to calculate the binding energy
you have to use the below formula :<br>
E_{binding-ZnO}=E{calculated-ZnO-wurtzite}-E_{Zn_{crystal-total}}-E_{E(1/2*O2}}.<br>
Remember that you have to do spin-polarised calculation
for O2 otherwise it will give wrong answer.<br>
</blockquote>
</div>
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?<br>
<br>
Furthermore, for ZnO wurtzite, should I do spin-polarised
calculation or not?</blockquote>
<div>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. <br>
</div>
</div>
</blockquote>
<br>
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: <br>
<br>
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, <br>
if we want to obtain the atom energy via DFT calculation by using
the big box model, a spin-polarised calculation should be used.<br>
<br>
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.<br>
<br>
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?<br>
<br>
<br>
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?<br>
<br>
Regards.<br>
<br>
<blockquote
cite="mid:BANLkTimNgBvuJ0FVinuXOX+FEN8pmaGxWw@mail.gmail.com"
type="cite">
<div class="gmail_quote">
<div> </div>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div class="im"><br>
<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> 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.<br>
</blockquote>
</div>
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?
<div class="im">Yea you have to use box model as you correctly
mentioned.<br>
<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> <a moz-do-not-send="true"
href="http://arxiv.org/ftp/cond-mat/papers/0610/0610002.pdf"
target="_blank">http://arxiv.org/ftp/cond-mat/papers/0610/0610002.pdf</a><br>
<a moz-do-not-send="true"
href="http://prb.aps.org/pdf/PRB/v70/i5/e054104"
target="_blank">http://prb.aps.org/pdf/PRB/v70/i5/e054104</a><br>
</blockquote>
<br>
</div>
Regards.<br>
<font color="#888888"> -- <br>
<br>
Hongsheng Zhao<<a moz-do-not-send="true"
href="mailto:zhaohscas@yahoo.com.cn" target="_blank">zhaohscas@yahoo.com.cn</a>><br>
School of Physics and Electrical Information Science,<br>
Ningxia University, Yinchuan 750021, China<br>
<br>
</font></blockquote>
</div>
<br>
<br clear="all">
<br>
-- <br>
Regards,<br>
MOHNISH,<br>
-----------------------------------------------------------------<br>
Mohnish Pandey<br>
Y6927262,5th Year dual degree student,<br>
Department of Chemical Engineering,<br>
IIT KANPUR, UP, INDIA<br>
-----------------------------------------------------------------<br>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Hongsheng Zhao <a class="moz-txt-link-rfc2396E" href="mailto:zhaohscas@yahoo.com.cn"><zhaohscas@yahoo.com.cn></a>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China</pre>
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