[Pw_forum] Elastic Constants

[Ben Palmer]

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