Dear Youssuf Sorry, I do not fully understand your question, so I try to answer but it might not be what you expect. in my experience it is better to use a supercell to avoid spurious interactions between periodically replicated images of the U-perturbed atom. The response to the alpha perturbation is generally linear around the converged value (U_in=U_out), but when U_in is far from the converged value you may found a weird behavior. If you have a good U_out value coming from a unit-cell calculation and you are looking for a converged value with respect to the cell dimensions, then use such U_out value as U_in value in a supercell calculation (with k-points scaled to sample in the same, or very close way the Brillouin zone). You should see a shift of the new U-out value, depending on the different interaction between perturbed atoms in PBC. Try again to start with such U_out as U_in until you reach convergence (0.1 eV is generally enough for standard purposes). Then you may try again to extend the supercell and to check if your last U_out value is stable in the new system... HTH
Giuseppe On Sunday, February 15, 2015 12:23:37 PM Youssef wrote: > Dear Giuseppe, > > Thank you very much for your help, I was really stuck. > > I have another question about the linear response approach, in fact I > successfully got the liner behavior of alpha=f(dn) and alpha=f(dn0) on > conventional cell, with a very good value of U with respect to the > literature, now when I move to supercell (2x2x2) the linearity is > broken !!! > > Any idea about this issue? > > P.S: The convergence criterion's are well tested in addition to cell and > atomic positions are relaxed ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ègaux en droits. Les distinctions sociales ne peuvent être fondèes que sur l'utilitè commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la libertè, la propriètè, la sùretè et la rèsistance à l'oppression. ******************************************************** Giuseppe Mattioli CNR - ISTITUTO DI STRUTTURA DELLA MATERIA v. Salaria Km 29,300 - C.P. 10 I 00015 - Monterotondo Stazione (RM) Tel + 39 06 90672836 - Fax +39 06 90672316 E-mail: <[email protected]> http://www.ism.cnr.it/english/staff/mattiolig ResearcherID: F-6308-2012 _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
