[Pw_forum] small energy difference in B1 and B2 structure

[Stefano Baroni]

On Jan 1, 2009, at 4:22 PM, Bipul Rakshit wrote:

> Dear PWscf user,

Dear Bibul Rakshit:

> First of all Happy New year to all of you.

all the same to you!

> I am doing electronic calculation of ScSb in B1 and B2 phase. From  
> the calculation, I am getting the following results.
>
> ScSb(B1)    a0=11.019 Bohr, E0= -290.2578 Ry
>
> ScSb(B2)    a0=6.834 Bohr, E0= -290.20279 Ry
>
> As you can see the energy difference is very small for large volume  
> compression.

I do not quite understand the question: why do you say that there is a  
"large volume compression"? There is none: 11.019 au  
(volume=11.019^3/4=334.5) is rather close to the experimental lattice  
parameter that I have been able to find in the literature (11.06),  
while the volume that you report for the B2 stucture (6.834^3=319.2)  
is certainly smaller, but certainly not "very compressed". Isn't it by  
any chance because of the _very_ different lattice parameter? If so,  
have you doble checked in you preferred solid state textbook the  
formules that give the unit-cell volumes for the B1 (FCC) and B2 (SC)  
structures?

> Why this happen? The input file for B1 and B2 is attached.

I was not able to detect anything manifestly wrong in your input  
files, assuming that you have carefully checked the convergence of  
your results wrt the number of k points, as needed particularly in  
view of the rather small value for the Gaussian broadening that you  
are using.

take care
SB

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale  
de la pensée - Jean Piaget

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html






-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20090101/3159afd4/attachment.html>