Dear All:
I used ph.x to calculate the phonon frequencies at GAMMA point of Ta4Pd3Te16 (its structure can be found at http://www.springermaterials.com). I had relaxed the structure so that all forces was little than 1mRy/bohr. The first time I used parameters : ... conv_thr=1.0d-7 ecutwfc=40 ecutrho=200 ... in self consistent calculation and parameters: ... tr2_ph=1.0d-12, ldiag = .true. start_irr = 1 last_irr = 33 ... / 0 0 0 in phonon calculations. Note that irrep1-irrep22 are Ag and irrep23-irrep33 are Bg (which are raman active), the rest irreducible representations are not raman active. I got a reasonable results which can be comparable with experiment but with an exception that the lowest frequency was imaginary: freq ( 1 - 1) = -28.6 [cm-1] --> A_g R freq ( 38 - 38) = 30.0 [cm-1] --> A_g R freq ( 39 - 39) = 36.3 [cm-1] --> B_g R ... Therefore I enhanced my precision as: ... conv_thr=1.0d-8 ecutwfc=40 ecutrho=300 ... ------------------ ... tr2_ph=1.0d-14, ldiag = .true. start_irr = 1 last_irr = 33 ... / 0 0 0 I expected to get a better result, but actually I got a worse one: freq ( 1 - 1) = -504.0 [cm-1] --> A_g R freq ( 2 - 2) = -333.9 [cm-1] --> A_g R freq ( 3 - 3) = -251.0 [cm-1] --> A_g R freq ( 4 - 4) = -140.0 [cm-1] --> A_g R freq ( 41 - 41) = 44.2 [cm-1] --> A_g R ...... I completely can't understand why. Can anybody help me ? Thanks -- Zhida Song Institute of Physics Chinese Academy of Sciences P.O. Box 603, Beijing 100190, China
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