Learn how to ask questions correctly

Alex Granovsky

gran@classic.chem.msu.su

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.

>>>

>>>

>>>Regards,

>>>

>>>

>>>

Fri Jul 31 '15 0:28am

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