[Pw_forum] Elastic Constants

[Mohsen Modaresi]

Dear Ben,
If you think you are  new in this topic, you need to study more references.
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.
Good luck.


On Fri, Nov 16, 2012 at 9:39 AM, Ben Palmer <benpalmer1983 at gmail.com> wrote:

> Hi,
>
> I've read a number of forum posts about calculating elastic constants,
> but I was hoping someone could give me some advice.  (I'm trying to
> calculate the elastic constants of bcc Iron).
>
>  From what I have read, there are two ways I could do this:
>
> 1) Relax the unit cell with vc-relax, deform the cell (by about 0.5-1%)
> and relax internally with relax.  I'd then have the stress tensor of the
> deformed cell.
>
> 2) Deform the relaxed cell at regular strain intervals, then fit a
> polynomial to the strain-energy data.
>
> Am I right in saying I have those two options, or am I partly or
> completely wrong?
>
> If I tried the first option, it's been said to calculate the elastic
> constants from the strain-stress relationship.  Would that mean solving
> the equation:
>
> Tij = Cijkl Ekl  the ijkl are subscripts, T stress, E strain, C
> stiffness tensor
>
> I'm fairly new to the subject so I could be quite wrong in what I've said.
>
> Thanks
>
> Ben Palmer
> Student @ University of Birmingham UK
>
>
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-- 
Mohsen Modarresi,
PhD student of Solid State Physics, Ferdowsi University of Mashhad, Iran.
Phone +98-9133452131
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