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Can small imaginary frequencies be ignored?

Siddheshwar Chopra

Dear All,
This is a very difficult situation. I have almost tried all the suggestions on this forum to get rid of imaginary frequencies.. But unfortunatley none have worked. I have checked the structure too.
I wish to ask that can we ignore VERY small imaginary frequencies? I have the following ones:

FREQUENCY:        62.07 I     21.98 I     20.05 I     14.22 I     10.76 I      8.02 I      6.86 I  

And in another run:

23.34 I     22.38 I     17.98 I     15.48 I     13.09 I     10.33 I      3.18 I

I read somewhere that low frequencies (especially first or second line) are not vibrations. They should be zero, but are not, due to numerical errors.

Is it correct? Can we safely ignore them? Will the DFT/TDDFT calculations be affected adversely?

In general, is there a thumb rule that we can ignore small frequencies? If yes, then how to decide till which value?

Someone please help.


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