[Pw_forum] Imaginary frequency in phonon spectrum calculation

[潭影空人心]

Dear all,
    With tr2_ph=1.0d-12 and q-points 2X2X2, there is no imaginary frequency among the calculated q points, and the frequency of GAMMA point is about 36 cm-1. After dynamic matrix calculation with  asr='crystal' and 100  interpolated points, I obtained  good phonon spectrum, but if I interpolated 200 points, a small imaginary frequency near GAMMA point appeared.

    Then I reduced tr2_ph to 1.0d-14,  unfortunately, the frequency of GAMMA point is about -6 cm-1, but  after dynamic matix calculation with asr='crystal' and 1000  interpolated points, it vanished and there is no imaginary frequencies at other points. However, 2000 interpolated points in dynamic matrix calculation courted imaginary frequency near GAMMA point again!
     I hope someone can help me with this promblem.
     Thank you very much!
Tan Lin
2015.10.29
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