Dear Prof. Baroni Thanks again
> Dear Prof. Baroni > > Thanks for your reply. > I wonder shouldn't the plot show the same frequency for a range of > displacements? > > > what range? why should it? > > -- Because if we are in the neighberhood of a minimum "where a quadratic could be fitted", any displacement in that range should result the same frequency. By displacement, I mean the same pattern but different in amplitude. > And the same freq. for the range to be the harmonic frequency? or the zero > displacement limit, is the harmonic frequency? > > > yes, by definition > > And when I test the displacements with the same amplitude, but different > in sign (regarding the inward and outward atomic movements in the basis) I > do not get the same results. > > > why should you? > > Shouldn't the plot have symmetry vs. the vertical axis? > > > why? > -- Consider for example a linear chain of atoms, with 2 atoms per basis. The optical mode is the result of the outward movement of the atoms (The one in +a/2 moves towards +x and the one in -a/2 moves towards -x). I expect that if I displace the atoms in an inward movement with the same amplitude as the outward, I should see the same frequency (The one in +a/2 moves towards -x and the one in -a/2 moves towards +x) because the 2 patterns present the same mode, right? That is what I've done in the case of Diamond. Is my expectation wrong? > > please, try you best to answer my questions, and I will make my best to let > you understand > > SB > > Mohaddeseh > Thank You again > Mohaddeseh > > > On Mon, Aug 1, 2011 at 6:33 PM, Stefano Baroni <baroni at sissa.it> wrote: > >> Hi Mohaddesh. I do not understand why you say that the calculated >> frequency (which, I assume you define as 2 * Delta E / u^2 ???) does not >> converge as u->0. According to your plot, it does, and to a reasonable >> value. The finite slope of your plot around u=0 seems to me nothing but a >> manifestation of cubic anharmonicity ... What's wrong in this? SB >> >> On Aug 1, 2011, at 3:01 PM, mohaddeseh abbasnejad wrote: >> >> >> Dear users, >> >> I have plotted the harmonic frequency (obtained within the frozen phonon >> approach) of Diamond as a function of atomic displacements (small >> displacements in the [1 1 1] direction, u(1)=?(u,u,u) and >> u(2)=?(-u,-u,-u) are the displacements of two atoms in the basis). I expect >> to see a range of displacements that tend to the similar frequency, but >> there is no convergence in the plot. Also, the displacements with the same >> amplitude but different in sign, do not tend to the same frequency, which is >> not what I expect, too. The plot is attached. >> I was wondering if anyone could help me to find the source of error. I >> should expect the mentioned symmetry, right? Why there is no convergence? >> >> Thanks in advance. >> >> Yours, >> Mohaddeseh >> --------------------------------------------------------- >> >> Mohaddeseh Abbasnejad, >> Room No. 323, Department of Physics, >> University of Tehran, North Karegar Ave., >> Tehran, P.O. Box: 14395-547- IRAN >> E-Mail: m.abbasnejad at gmail.com >> Website: http://physics.ut.ac.ir >> >> --------------------------------------------------------- >> >> <fig..pdf>_______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> >> >> --- >> Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - >> Trieste >> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / >> stefanobaroni (skype) >> >> La morale est une logique de l'action comme la logique est une morale de >> la pens?e - Jean Piaget >> >> Please, if possible, don't send me MS Word or PowerPoint attachments >> Why? See: http://www.gnu.org/philosophy/no-word-attachments.html >> >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> >> > > > -- > --------------------------------------------------------- > > Mohaddeseh Abbasnejad, > Room No. 323, Department of Physics, > University of Tehran, North Karegar Ave., > Tehran, P.O. Box: 14395-547- IRAN > Tel. No.: +98 21 6111 8634 & Fax No.: +98 21 8800 4781 > Cellphone: +98 917 731 7514 > E-Mail: m.abbasnejad at gmail.com > Website: http://physics.ut.ac.ir > > --------------------------------------------------------- > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > > --- > Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste > http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / > stefanobaroni (skype) > > La morale est une logique de l'action comme la logique est une morale de la > pens?e - Jean Piaget > > Please, if possible, don't send me MS Word or PowerPoint attachments > Why? 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