Stefano, Nicola, Thank you for your comments -- this is some good food for thought! Making electrodes an explicit part of the calculation will surely be necessary in the case of thin-ish films, but it opens its own big can of worms (band alignment between components with DFT band gaps, etc; I bet you know the issues very well) I do not want to deal with in this particular project. For the case of thick-ish films with assumed perfect screening I could probably guesstimate things around Gamma by taking an "honest" bulk case as a reference and then playing with phonon postprocessing routines (easy, since no new time-consuming DM calculations are required).
But, I guess, the thought of how to add screening to this process in a more rigorous fashion is worth some additional thinking. THX, Serge Stefano Baroni wrote: > Dear Serge: > >> >> I understand that in bulk calculations the dynamical >> matrices at small q are split into analytic and >> nonanalytic parts, with the latter accounting for the >> effects of macroscopic electric field. However, if >> we want to study phonons in a model representing an >> electroded thin film instead of a bulk crystal (i.e., >> the electrodes are not explicitly included in such a >> calculation), we should assume that such an electric >> field will be screened out by the free charges and >> its influence onto ionic vibrations should not be >> considered. >> >> I am under an impression that to achieve this in pwscf >> I need to block the calls to nonanal(...) subroutine >> in dynmat and matdyn, while leaving everything else >> intact. Would this work out or could it possibly be not >> as trivial as I imagine it? > > I am afraid it is indeed not as trivial as you imagine it ... I am no > longer familiar with the guts of the code, but the only thing the code > can possibly do is to make the phonons calculated at q=0 equal to the > values that the phonon branches would have in the long-wavelength limit > (and those depend in a non trivial way on the combination of > wavevector/polarization, because of the long range character of the > Coulomb interaction). I think that, in the presence of free carriers, > the results of the code are in principle correct whenever the phonon > wavelength is smaller that the screening length. At q=0 just setting the > effective charges equal to zero (which is what you would presumably get > by commenting out some appropriate routine calls) should do the job, but > you would then get frequencies that are inconsistent with those > calculated at finite q. To get fully consistent results, I think that > screening from free carriers should be explicitly accounted for in the > calculation of the dynamical matrix. Possible, but "not as trivial as > one could imagine". > > Cheers - stefano > -- ********************************************************* Serge M. Nakhmanson phone: (630) 252-5205 Assistant Scientist fax: (630) 252-4798 MSD-212, Rm. C-224 Argonne National Laboratory 9700 S. Cass Ave. Argonne, IL 60439 *********************************************************
