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

[Lorenzo Paulatto]

On Thursday, December 17, 2015 03:31:02 PM yin li wrote:
> 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.

In principle you should get exactly zero, it is a mathematical condition which 
is quite straightforward to impose. I only saw non-zero frequency after sum 
rule when also effective charges are present.

In any case, these are low enough to be considered zero, except in some very 
specific corner case.

> # 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.

Is your system an isolated molecule, or a nanocluster? Because the zero-dim 
acoustic sum rule is only suited for these cases, and must no be used for 
anything else.



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Dr. Lorenzo Paulatto
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