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Re^3: Can small imaginary frequencies be ignored?

Alex Granovsky

Dear Siddheshwar,

visualize these frequencies in ChemCraft and then decide if they are
T+R modes or not. Look at the Satvetz condition numbers, this will
help you to decide. If these modes are not T+R modes, try to decide
if they can be safely neglected for your purposes.

Kind regards,
Alex Granovsky

On Wed Jul 29 '15 2:22pm, Siddheshwar Chopra wrote
>Dear Alex,
>Thank you for the information. I get your point. But IS the second run fine? It shows 23.34 I as maximum frequency. Is this fine?

>Also, there are NO real imaginary frequencies obtained in this case..

>Kind Regards,
>On Wed Jul 29 '15 0:43am, Alex Granovsky wrote
>>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.

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