RE: [SIESTA-L] Band structure of MoSe2

[Salvador Barraza-Lopez]

Dear Xiaoming,

I've read your last e-mail, seen the bands for bulk Mo, and ran out of 
suggestions. Hopefully other members have already worked on the system and can 
provide more detailed input. Your structures look quite symmetric already.



 Best regards,

-Salvador






From: siesta-l-requ...@uam.es  on behalf of Xiaoming 
Wang 
Sent: Wednesday, May 6, 2015 2:49 PM
To: siesta-l@uam.es
Subject: RE: [SIESTA-L] Band structure of MoSe2

Dear Salvador,

Thanks so much for your advice and comments! I have compared the band structure 
of bulk Mo from ELK and Siesta with different pseudos (see the attached 
figure). In the figure, CA_07 is the pseudo from 
http://charter.cnf.cornell.edu/psp_files/Mo.psf,
 CA_12 from 
http://departments.icmab.es/leem/siesta/Databases/Pseudopotentials/Pseudos_LDA_Abinit/Mo_html/Mo.psf,
 and I also tried the pseudo by A. Khein and D.C. Allan as you suggested, the 
result of which is very close to that of CA_12, thus, I don’t show in the Fig.  
As seen from the Fig, I think both of them are acceptable as for bulk band 
structure calculations, but there are some discrepancies about the lattice 
constant. The energy shift of 50 meV was chosen for the calculations, I also 
tried 10 meV of ES, but nearly no significant change can be found. I didn’t 
test the bulk Se calculations, as you have done this.  The relaxed structure is 
attached below which is calculated by using CA_07 of Mo pseudo and Se pseudo 
from Comput. Mater. Sci. , 98 (2015) 372-389,  I think it is 
hexagonal-symmetric, but the K point bands problem still not fixed. And I 
cannot understand how to re-symmetrize the relaxed structure, can you please 
explain a little more? By the way, as you mentioned black phosphorus, I also 
did this kind of calculations before, the problem is not only the gap change 
with some strain as you said but also there are some split about the bands, it 
seems that I turned on the SOC during the calculation. Is this also the result 
of not proper pseudos?

Structure of MoSe2:
  3.248660546   0.0   0.0
 -1.624330273   2.813422563   0.0
  0.0   0.0  13.0
   3
  142   0.4   0.4   0.505632878
  234   0.7   0.2   0.632853521
  234   0.7   0.2   0.378419220


Best regards,

Xiaoming



From: siesta-l-requ...@uam.es [mailto:siesta-l-requ...@uam.es] On Behalf Of 
Salvador Barraza-Lopez
Sent: Friday, May 1, 2015 10:08 AM
To: siesta-l@uam.es
Subject: RE: [SIESTA-L] Band structure of MoSe2


Dear Xiaoming,

Okay, I see. I have some further comments.



You set a relaxation with cell parameters evolving: That setting could be 
responsible for shifting valleys away from the K-point (this similar to the 
effect of K-point shifting induced by strain; this has been discussed 
extensively on other materials with similar structural symmetries and where the 
K-point is special, such as graphene; this is a known effect). So you want to 
re-symmetrize the relaxed structure, especially if the lattice vectors change. 
This may fix the issue near K.



In addition, there is no Mo pseudo on the article you mention, only Se. WSe2 is 
shown as Fig 15 there. Note that there are some discrepancies with VASP results 
on that Figure, and on the bands for W in Figure 10 there as well, but they are 
not nearly as dramatic as the ones you show for MoSe2.



Once your MoSe2 structures are strictly hexagonal-symmetric and fine, may I 
suggest that you compare lattice parameters and band structures for bulk Mo 
with SIESTA and with another code (ELK or Wien2K -all electron codes- could 
work for this purpose). You may want to do the same for Se (we did this for Se 
on the paper you mention, and the datasets are attached as a PDF file here: 
http://www.sciencedirect.com/science/article/pii/S2352340914000353
 ; we did not work with Mo though).



 Here, you have to re

[SIESTA-L] Dielectric function

[Suman Chowdhury]

Dear User,
I have a question. By definition the imaginary part of the dielectric
function is positive and also it can be seen from its mathematical
expression that at zero frequency it should be zero. Right now I am
studying some structure where sometimes I am getting 0 at 0 frequency and
sometimes I am not getting 0 at 0 frequency. It is coming of the order of
10. Do any of you know what is the reason behind it.

-- 



*Senior research fellow Dept. of Physics, University of Calcutta Kolkata-
79, West Bengal, India.*
* Ph no-+91-9830512232*