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

[Romeda Azeen]

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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