Electric direction for 2-D anisotropic systems

[huangj3]

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

[Daniele Varsano]

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