On Thu, 13 Jul 2006, bipul rakshit wrote: | hello siesta user, | i am doing the phonon calculation with the help of Vibra from the siesta2.0 package. | i am doing PtN with LDA approximation. But in the dispersion plot only two modes are seen. Why not all the modes are visible. | i am sending the input files used in the calculation | ptn.fdf for fcbuild and vibra | and ptn-siesta .fdf for siesta | can any body have a look and tell me where i commit mistake
Bipul: as you give no output, I can only guess from your vibra input that you only get Gamma phonon because you don't define SuperCell and hence you won't get any phonon dispersion. You define many q-values for the plot, but they will probably give you always the same numbers for the frequency, because you don't have any information in your calculation about how the force constants change in the real space from neighbor to neighbor, and this is crucial to get the phonon dispersion, i.e. to Fourier-transform this real-space dependence of the force constants. Still, you can already check from your calculation whether your Gamma phonon is OK - i.e., you must get three nearly zero numbers and three nearly-equal numbers, which would the the triply degenerate Gamma-TO phonon in PtN; you can certainly find a reference to compare. If this is OK, and you need to calculate phonon dispersion, you'd need to construct a supercell large enough, i.e. SuperCell_1,2,3 > 0 to assure that the force constants fall down within this supercell. You ask about instructions to vibra - the documentation file is in Util/Vibra/Docs Best regards, Andrei +-- Dr. Andrei Postnikov ---- Tel. +33-387315873 ----- mobile +33-666784053 ---+ | Paul Verlaine University - Institute de Physique Electronique et Chimie, | | Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France | +-- [EMAIL PROTECTED] ------------ http://www.home.uni-osnabrueck.de/apostnik/ --+
