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Re^3: Can small imaginary frequencies be ignored?

Jim Kress

From the documentation:

Further Information 4-155

The RUNTYP=HESSIAN's normal coordinate analysis is rigorously valid only at stationary points on the surface. This means the frequencies from the hessian at your trial geometry are untrustworthy, in particular the six "zero" frequencies corresponding to translational and rotational(T&R) degrees of freedom will usually be 300-500 cm**-1,
and possibly imaginary.

The Sayvetz conditions on the printout will help you distinguish the T&R "contaminants" from the real vibrational modes.

If you have defined a $ZMAT, the PURIFY option within $STATPT will help zap out these T&R contaminants). If the hessian at your assumed geometry does not have one and only one imaginary frequency (taking into account that the "zero" frequencies can sometimes be 300i!), then
it will probably be difficult to find the saddle point. Instead you need to compute a hessian at a better guess for the initial geometry, or read about mode following.

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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