[QE-users] info about setting celldm

[patrizio.graziosi at cnr.it]

Hello everybody,

I'd like to have from you a proof/disproof of my understanding on the  
use of celldm for F space groups.

I read in the instructions that QE uses the primitive cell, in the  
case of Silicon I mean for that the "slanted" cell having parameter  
around 0.387 nm and 60° angles, say like here  
https://www.iue.tuwien.ac.at/phd/ungersboeck/node27.html .

However, in an example for silicon, here  
https://www.quantum-espresso.org/resources/tutorials/shanghai-2013/getting-started/lecture1.pdf , I see that in the case of silicon with ibrav = 2 the entered celldm correspond to 0.543 nm, the side of the "cubic  
cell".

Thus, if I'm working with a F space group, ibrav = 2, I must use the  
cube side as celldm, ~ 0.54 nm for silicon. Is this correct?

If I'd like to work with the "slanted primitive cell" with angles of  
60°, which in silicon has lattice constant ~ 0.387 nm, how can I do? I  
apologize but I couldn't find this in any tutorial I found.

Thanks for your time,
Patrizio

-- 

Patrizio Graziosi, PhD

Research Scientist

CNR - ISMN
Institute for the Study of Nanostructured Materials