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** Re: Can small imaginary frequencies be ignored? **
Alex Granovsky

gran@classic.chem.msu.su

Dear Siddheshwar,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,

>

>

>

Wed Jul 29 '15 0:43am

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