[Pw_forum] Determining the value of emaxpos and eopreg

[Giovanni Cantele]

emaxpos is the position of the superimposed electrostatic potential jump, in the direction normal to the surface, as specific by edir. It is in the units of the lattice vector in the edir direction, so its value ranges from 0 to 1. emaxpos MUST specify a position in the vacuum separating periodic replicas of the slab, because the potential jump corresponds to an artificial trick used to simulate a constant electric field within periodic boundary conditions. If there where an overlap of the region where the charge density is significantly different from 0 with the position emaxpos, your total energy would receive an unwanted and unphysical  contribution from such a jump in the potential.
Example: let us suppose that edir = 3, that the c axis is orthogonal to the other two and that the z coordinates of your atoms range from z0 to z1, in units of c. PROVIDED YOU HAVE CHOSEN A REASONABLE (that is, not so small) VALUE OF THE VACUUM SPACE, the nearest (along edir) atom(s) of the atoms at z1 are at z0+1. That means that the vacuum space ranges from z1 to z0+1. You can choose emaxpos = (z0+1+z1)/2, so that the jump is located exactly in the middle of the vacuum.


eopreg specifies instead the jump width, again in units of the lattice vector along the  edir direction. You can choose the default value (0.1) or oven smaller (e.g. 0.05), because you want that this unphysical jump does not involve a wide range along edir, again because you want to minimise (and , eventually, to get rid of) a possible overlap of the charge density with such a region.

Giovanni

PS I’m forwarding this message also to the forum, because it is usually much more efficient to ask help to many people that to just one person! Please reply to the forum if you have any further request….

> On 1 Dec 2017, at 08:29, Mohammed Kader Ghadiyali <ghadiyali.mohd at physics.mu.ac.in> wrote:
> 
> Respected Sir,
> 
> I am currently trying to do compute band structure with saw tooth potential applied in the z-direction, but I am not able to determine the value of "emaxpos" and "eopreg". The only reference I could find is a PW forum thread [1]. I am working on a hexagonal system, can you inform me the procedure to determine the values of emaxpos and eopreg. I have tried the example given with QE.
> 
> Regards,
> -- 
> Ghadiyali Mohammed Kader
> Research Scholar
> Department of Physics
> University of Mumbai
> +91 8898519951

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.fisica.unina.it/~cantele

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