Dear Catrina, Mind that you have periodical boundary conditions so your unit cell is repeated infinite number of times in all directions. So, you only need to construct a supercell if you want to break the symmetry (i.e. introduce defects). Or if you want to imitate zero boundary conditions (i.e. for isolated molecules, surfaces etc) by constructing a large supercell with a vacuum layer.
if you calculate just an ideal bulk crystal, making supercell does not change anything at all because of periodic boundary conditions! so you just waste the computation time if you do it. regards, Yurko 2008/11/25 catrina desport <[EMAIL PROTECTED]>: > Dear Andrei Postnikov, > > I am sorry if my words were confusing. Actually, it is not the matter of > imitating at all. I just wanted to know if having supercell for some systems > like bulk hexagonal helps to get better answers. > > I didn't enclose their article to keep it from judgements, but in a > presentation I found from the work they did, I saw the below lines: > > Bulk ... (hexagonal system) > SIESTA: TM pseudopotential, ... basis, ... Ry mesh cutoff, LDA > Piezoelectric constant > Berry Phase: e33=... C/m2 > Our O(N) method: e33=... C/m2 > 6×6×2 supercell > > Again, I insist, it is not the matter of imitating at all, just wanted to > know if there's a helpful method to get reasonable parameters for hexagonal > systems, especially those which are abit unstable. > > I hope to have your valuable response. > Regards, > Catrina > ________________________________ > From: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> > To: SIESTA-L@listserv.uam.es > Sent: Tuesday, November 25, 2008 6:32:02 PM > Subject: Re: [SIESTA-L] URGENT (supercell for bulk) > >> To all respected siesta users: >> >> Considering >> an article on a hexagonal lattice, written by authors who have worked >> with siesta, I have encountered that they have used 6x6x2 supercell for >> their "bulk" system !!! > > Dear Catrina: > if it is really supercell used in their calculation and not just > superc: Internal auxiliary supercell > message, then they should have had a good reason for doing it > (usually, modeling a point defect, ....) and not just bulk? > >> I need to know how they have done it to >> generate the appropriate supercell > > I'd guess, rather by hand or by using the SuperCell block > (once, and then exporting the generated coordinates in the input file) > >> for better optimization for my >> system as well. > > I wonder how can supercell help in "better optimization of your system"? > >> Please do guide me, it is very urgent for me. > > This is difficult for two reasons: > you do not explain what you want to do, > and refer to other peoples' work which you apparently want to imitate, > without giving any details or reference. > > Best regards > > Andrei Postnikov > > -- Yurko (aka Yuriy, Iurii, Jurij etc) Natanzon PhD Student Henryk Niewodniczan`ski Institute of Nuclear Physics Polish Academy of Sciences ul. Radzikowskiego 152, 31-342 Krako`w, Poland Email: [EMAIL PROTECTED], [EMAIL PROTECTED]
