Hi Eyvaz, I am a little bit lost with your answer, what has some "real" instability at (100) to do with some "numerical" instability close to Gamma?
Best, Katalin > Hi, > > If I understood correctly, you have got imaginary (but > not negative!, as you mentioned) frequences for X(100) > point using 4x4x4 q-mesh. If so, the use of 6x6x6 mesh > which also has (100) point is hopeless. > Appearance of imaginary frequency at (100) point is a > usual case for unstable compounds within NaCl > structure. > > I suggest, it is somewhat strange to do FFT (q2r.x) > having an imaginary frequency and then expect a stable > structure. These imaginary frequences near Gamma point > you found using q2r.x, are, presumably, connected to > X(100) point instability. > > So, I guess, the problem might be due to: > a) low cutoff energy > b) insufficent k-points mesh > c) quality of a pseudopotential > d) check carefully your input file > > Best regards, > Eyvaz. > > > --- li yan <liyanpcl at yahoo.com.cn> wrote: > >> Dear katalin, >> >> I want to find the reason of the phase >> transiton for AgF by calculating the phonon >> dispersion curves. My intent is finding the soft >> mode at high pressure. >> >> >Which was the first grid, which you used to >> calculate the dispersion? >> >> AgF is fcc strcture, c/a is 1, so the mesh was >> initially chosen as 4*4*4. >> >> At 3GPa, after calculating the eight special >> points, i found only one of them (0,0,1) has a >> negative mode. Then i did the calculation of the >> phonon disperion curves with q2r.x, the frequences >> of TA near the gama point are negtive. Near gama >> point, i just selected one q, whose TA is negtive, >> to calculate the phonon, but the value of the TA is >> positive. I thought the q mesh is small, so i >> chose 6*6*6 mesh. >> >> > Does the 6x6x6 mesh include also the points you >> calculated for checking. >> >> The 6*6*6 mesh dosen't include the above >> modes which has negtive values, but i cannot find >> other ways to avoid the unexpected negtive values. >> >> thank you for your advice. >> best regards >> Li Yan >> >> >> >> >> >> >> >> --------------------------------- >> ???????????????????????????????????????????????????? > > > > > > __________________________________________ > Yahoo! DSL ? Something to write home about. > Just $16.99/mo. or less. > dsl.yahoo.com > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum >
