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