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Hi Mohsen,<br>
<br>
I will try as you suggest and see if I can calculate the constants
using the second derivative of energy.<br>
<br>
All the best,<br>
<br>
Ben<br>
<blockquote
cite="mid:CAM2BXVARTdzTyWWoWkRghKgR=bUS_QTot+g1BTUMH+nT7MdoGw@mail.gmail.com"
type="cite">Dear Ben,<br>
If you think you are new in this topic, you need to study more
references.<br>
As you said you have two specific ways. I only share my experience
on this subject. The first way is not applicable for super-cell
model. For example you can calculate the Youngs moduls of Graphene
(which is a 2D material) by using this method. by
increasing/decreasing the space between layers the elastic
constant changes (which has not any physical reason). So i suggest
the second way. You could vc-relax the structure, then deform it
and relax by using "relax". By using the second derivative of
energy with respect to the strain Y is calculated.<br>
Good luck.<br>
<br>
<br>
<div class="gmail_quote">On Fri, Nov 16, 2012 at 9:39 AM, Ben
Palmer <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:benpalmer1983@gmail.com" target="_blank">benpalmer1983@gmail.com</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;">
Hi,<br>
<br>
I've read a number of forum posts about calculating elastic
constants,<br>
but I was hoping someone could give me some advice. (I'm
trying to<br>
calculate the elastic constants of bcc Iron).<br>
<br>
From what I have read, there are two ways I could do this:<br>
<br>
1) Relax the unit cell with vc-relax, deform the cell (by
about 0.5-1%)<br>
and relax internally with relax. I'd then have the stress
tensor of the<br>
deformed cell.<br>
<br>
2) Deform the relaxed cell at regular strain intervals, then
fit a<br>
polynomial to the strain-energy data.<br>
<br>
Am I right in saying I have those two options, or am I partly
or<br>
completely wrong?<br>
<br>
If I tried the first option, it's been said to calculate the
elastic<br>
constants from the strain-stress relationship. Would that
mean solving<br>
the equation:<br>
<br>
Tij = Cijkl Ekl the ijkl are subscripts, T stress, E strain,
C<br>
stiffness tensor<br>
<br>
I'm fairly new to the subject so I could be quite wrong in
what I've said.<br>
<br>
Thanks<br>
<br>
Ben Palmer<br>
Student @ University of Birmingham UK<br>
<br>
<br>
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</blockquote>
</div>
<br>
<br clear="all">
<br>
-- <br>
<div>Mohsen Modarresi, </div>
<div>PhD student of Solid State Physics, Ferdowsi University of
Mashhad, Iran.<br>
Phone +98-9133452131</div>
<br>
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