[Pw_forum] How to choose acoustic-sum-rule?

[yin li]

Dear QE community,

I wonder if there is a rule to choose acoustic sum rule. I set very tight
convergence criterion for both pw.x (conv_thr = 1.0D-10) and
ph.x(tr2_ph=1.0d-16).

When I applied asr="simple", the frequencies of the first 3 modes are not
equal to zero, but a relatively small negative value. The low-frequency
modes are affected, and there is no change in frequency number of
high-frequency modes.
# mode   [cm-1]    [THz]      IR
    1     -1.27   -0.0381    0.0000
    2     -1.02   -0.0306    0.0000
    3     -0.48   -0.0143    0.0000
    4     50.74    1.5212    0.0042
    5     66.71    1.9999    0.0031
    6     76.72    2.3000    0.1024
    7     91.30    2.7372    0.0605
    8     92.44    2.7712    0.2996
    9    102.29    3.0666    0.2928
   10    107.09    3.2105    0.0402
   11    113.55    3.4043    0.3288
   12    117.59    3.5252    0.5163
   13    133.35    3.9977    0.0394
   14    136.70    4.0981    0.2142
   15    144.03    4.3179    0.4547
   16    158.41    4.7490    0.0742


If I employed asr="zero-dim" , the frequencies of the first 6 modes all
become 0. But I found the frequencies of all the modes, even high-frequency
modes, were varied.

# mode   [cm-1]    [THz]      IR
    1     -0.00   -0.0000    0.0000
    2     -0.00   -0.0000    0.0000
    3     -0.00   -0.0000    0.0000
    4     -0.00   -0.0000    0.0000
    5     -0.00   -0.0000    0.0000
    6      0.00    0.0000    0.0000
    7     40.75    1.2216    0.0265
    8     68.45    2.0520    0.1827
    9     82.84    2.4836    0.1281
   10     95.99    2.8778    0.2143
   11    108.98    3.2671    0.3284
   12    113.25    3.3951    0.4248
   13    117.89    3.5342    0.2717
   14    126.27    3.7854    0.0136
   15    136.33    4.0872    0.3745

I am specially interested in the low-frequency region (1-4 THz). How could
I select acoustic sum rule correctly?

Thank you very much in advance!

Best Regards,
Yin
------------------------------
Dr. Yin Li
Department of Biophysics,Medical School, University of Pecs,
No.12 Szigeti Street, Pecs, H-7624, HUNGARY
Phone: +36-72-535271/36271
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