[Pw_forum] equilibrium lattice parameters

[Stefano Baroni]

On May 2, 2007, at 8:59 PM, Jess Kondor wrote:

>
> Hello,
>  I have a tetragonal unit cell with experimental lattice parameters  
> (a=b=/c). In order to find their theoretical values, I performed  
> variable cell relaxation (vc-relax). But I want to be sure that I  
> performed these calculations correctly. I know it is possible to  
> perform several static calculations for different lattice  
> parameters (or volume) and then fit the data by EOS. But I only see  
> (for example, ev.f90 from espresso distribution), that it is  
> possible only for cubic and hexagonal unit cell. Could anyone tell  
> me can I use such kind of technique in case of tetragonal unit cell?

YES, with some extra effort. You have to optimize first c/a keeping  
the volume fixed. The discontinuities of the total energy as a  
function of c/a (due to the fact that the number of PWs change as you  
change c/a) are less critical than the discontinuities in E(V). Once  
you have E(V), you can fit it to an equation of state, much in the  
same way you would for a cubic system.

Stefano


>
>
> cheers,
>    Jess

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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