[Pw_forum] How to optimize a hexagonal closest-packed (hcp) structure?

[Stefano Baroni]

The best method is the one that you find so by direct experience, because this would teach you something that you will never learn by asking around questions that you could answer by yourself. (it would also help you clarify what you mean by "best" bethod).

If you had tried by yourself, most probably you would have found that the best method is (3), because it is the one for which the number of plane waves depends the least on the parameter you vary.

Questions for you:

1) is this really so?
2) why would it be best to keep the number of plane waves "as constant as possible"?

Best wishes,
Stefano B
 
On Mar 31, 2012, at 4:39 PM, Romeda Azeen wrote:

> Dear QE developers, 
> I have a question about optimization of a hcp structure (e.g. Ti and Zr).
> Any hcp structure have two lattice constants (a and c), so for scf calculation, it is nessesry to optimize both lattice constants.
> 
> 
> Method 1: Fixed celldm(3) at ideal c/a ratio (1.633) and then changed celldm(1)=a and eventually computation of total energy. 
> 
> Method 2: Fixed celldm(1) at experimental lattice constant and then changed celldm(3)=c/a and computing of total energy.
> 
> Method 3: Fixed unit cell volume at experimental volume and then changed c/a ratio and calculation of a (Volume=fixed) and run scf 
> 
> calculation with different a and c/a. 
> 
> 
> what is best method for optimization of a and c? Method 1, 2 , 3 or others method?
> 
> 
> I am very thankful any to help me.
> 
> --
> Romeda Azeen,
> Bhavnagar University Bhavnagar 364002 Gujarat India
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+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

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