[Pw_forum] Elastic Constants

[Ben Palmer]

Hi Mohsen,

I will try as you suggest and see if I can calculate the constants using 
the second derivative of energy.

All the best,

Ben
> 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 
> <mailto: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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