Siddheshwar Chopra
sidhusai@gmail.com
Also, there are NO real imaginary frequencies obtained in this case..
Kind Regards,
On Wed Jul 29 '15 0:43am, Alex Granovsky wrote
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>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,
>>
>>
>>