Yambo Community Forum

Dear all,

I have a question regarding the electric field direction for a 2-D aniostropic system. If I invoke BSE by $ yambo -b -o b -y h -V qp, there are two lines associated with the electric field direction:
% LongDrXs
1.000000 | 0.000000 | 0.000000 | # [Xs] [cc] Electric Field
and
% BLongDir
1.000000 | 0.000000 | 0.000000 | # [BSS] [cc] Electric Field

I understand that for a 1-D system, both of these two lines should be modified so that the direction is along the chain direction. For a 2-D system in xy plane, for instance, should these two lines be changed to 1.000000 | 1.000000 | 0.000000 | ?

I am sort of confused by the tutorial of Si(111)2x1 example with a Pandey reconstruction where the electric field was modifed to
0.000000|1.000000|0.000000| instead of 1.000000 | 1.000000 | 0.000000 |.

Thanks a lot for your help.

Jingsong Huang
Computer Sciences and Mathematics
Center for Nanophase Materials Sciences
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA

Dear Jingsong Huang,
there are two variable because:
% LongDrXs
indicates the direction of \vec q->0 for the calculation of the screening dielectric matrix you need in the screened
coulomb interaction entering in the BSE, while
% BLongDir
refers to the q->0 direction in the calculation of the optical response, see eps2 definition here.
So, the %BLongDir gives you the direction you want to look the absorption, and may be you would calculate both the direction
in the plane of your 2D system, while
% LongDrXs, could also affect in the case of anysotropic system, but in a milder way. You can have a look to this paper:
PHYSICAL REVIEW B 77, 235428, 2008
and reference therein for a correct tensor treatment of the anysotropy, in particular refs. 36 and 37.

Hope it helps,

Cheers,
Daniele