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Alex Granovsky
gran@classic.chem.msu.su

In theory, there should be exactly 6 (or 5 for linear molecules)
rotation and translation (T+R) modes having zero frequency. In
practice, due to accumulation of various numerical errors or due to
approximate nature of calculations these zero-frequency modes can be
small positive or imaginary numbers. Indeed, sometimes very small
imaginary frequencies correspond to T+R modes. However, as your
systems have at least seven imaginary modes, all of them cannot be
rotations and translations because there are only six T+R modes.
You do have some real imaginary frequencies!

As to neglecting imaginary frequencies, there is no recipe here.
Some people neglects rotations of -CH3 group. I personally believe
that frequencies like 62.07*I cannot be neglected and you need
to re-optimize your structure.

Kind regards,
Alex Granovsky


On Mon Jul 27 '15 2:18pm, Siddheshwar Chopra wrote
--------------------------------------------------
>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.
>
>
>Regards,
>
>
>