## Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

** Re^2: Can small imaginary frequencies be ignored? **
Siddheshwar Chopra

sidhusai@gmail.com

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,

>>

>>

>>

Wed Jul 29 '15 2:22pm

This message read **608** times