[QE-users] Symmetry - Centered cell reduction

[Paolo Giannozzi]

I am not sure I understand your problem. For face-centered and 
body-centered lattice, one may use a minimum-size "primitive" cell or a 
larger but simpler  "conventional" cell, having a simple cubic (or 
simple tetragonal, or whatever applies) lattice. Is this what you are 
referring to?

QE uses primitive cells because it is _always_ convenient to use the 
smallest cell: the computation increases as (cell volume)^3, while the 
number of k-points decreases as 1/(cell volume).

If however you really want to use the conventional cell for whatever 
reason, it is easy to do that using "way 3)" you describe below. Just 
specify a simple cubic (simple tetragonal, whatever) lattice and all the 
atomic positions in the conventional cell.

It wouldn't be difficult to force "way 2)" to do the same automatically, 
but this requires a) adding an additional variable (not a big deal), and 
b) checking all 230 space groups and changing accordingly many of them 
(also simple but looong and error-prone)

Paolo

On 17/12/2022 04:39, Fnukal Frantisek via users wrote:
> Dear QE users,
> 
> 
> I've studied the possible ways of inputing structural data and I found there basically were 3 ways. 1) Using ibrav = 0, so in essence saying: "Find it yourself" 2) Specifying the space group 3) Specifying the Bravais lattice with ibrav.
> 
> So far I've been using ibrav = 0, however output files keep telling me this way is not recommended as it may not correctly recognize symmetry in some cases.
> 
> However, I have a very important problem with the other 2 ways of inputing data. I've noticed they both automatically perform transformation to primitive cell if I input a structure in centered cell. My problem is with k-points. If I want to have an equivalent amount of k-points in the primitive cell, then I have a smaller system, but more k-points and in some cases this may lead to completely erasing any profit gained by reducing the system. I would very much like to control whether the transformation is applied or not.
> 
> My question is this: Is there any way of inputing structural data, preferably with space group, and turn off automatic centered cell to primitive cell conversion?
> 
> 
> Thank you for your answers.
> 
> 
> Best regards,
> 
> Frantisek Fnukal
> 
> UCT Prague
> 
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216