## Meaning of the tag LongDrXp

Various technical topics such as parallelism and efficiency, netCDF problems, the Yambo code structure itself, are posted here.
Longjun Xiang
Posts: 28
Joined: Tue May 07, 2019 9:53 am

### Meaning of the tag LongDrXp

Dear developers:

I have a technical question related to the tag LongDrXp: In your tutorials, the tag means direction of the electric field for the calculation of the q=0 component of the dielectric constant. However, in terms of your implementation paper:yambo: An ab initio tool for excited state calculations, the formula (6)-(9) (see https://www.sciencedirect.com/science/a ... 5509000472) are devoted to calculating the dielectric constant with PPA approximation. But where is the direction of the electrical field in these formulae? Can I just set (1,1,1) for bulk system and (0,0,1) for two-dimensional systems？ But what's the physical meaning of this direction?
Dr. Longjun Xiang
School of Physical Science and Technology, ShanghaiTech University, China

Daniele Varsano
Posts: 2149
Joined: Tue Mar 17, 2009 2:23 pm
Contact:

### Re: Meaning of the tag LongDrXp

Daer Longjun,
the electric field appears in the definition of dipoles, ie the rho_nn appearing in EQ.8.
You can have a look to the new yambo paper where this issue is explained in details:
https://iopscience.iop.org/article/10.1 ... 48X/ab15d0
See sec.3.1

Anyway, this is the direction of the electric field, or if you want the polarization of the photon perturbing your system.
Setting (1,1,1) for a bulk system is a good idea (you are somehow averaging in the 3 directions). For a 2D system (e.g. lying in xy direction) a good way to set the LongDrXp variable could be (1,1,0), i.e. light polarized in the plane of the system.

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/