About LongDrXp

[jmmorbec]

Hello.

I am working with an orthorhombic cell (a < b < c). And I noticed that the direct band gap changes when I change LongDrXp from 1.0|0.0|0.0 (default) to 1.0|1.0|1.0.
How should I decide about this variable? Should it be proportional to the lattice vectors?

I will really appreciate any help.

[Daniele Varsano]

Dear Juliana,
this variable indicate the direction of the q->0 limit in the response function to calculate W. If the system it is not strongly anysotropic the direction should not affect too much the final results. When dealing with 1D (wire), 2D (slab) etc., then it could ne an issue. For a discussion on the influence of that and anysotropic effects in slabs you can have a look to: Freysold et al. PHYSICAL REVIEW B 77, 235428 2008 and references therein. If you are dealing with bulk system it is strange the gap change considerably. In any case, a 1,1,1 direction, can be seen as a sort of average in the three spatial direction.

Best,
Daniele

[jmmorbec]

Thank you very much for your reply, Daniele.