Subject: CCL: Summary minimum geometry and imaginary frequencies

From: Pedro Aprigliano Fernandes <PFERNANDES.-at-.cetem.gov.br>
Date: Tue, 17 Feb 2004 16:01:20 -0300

Dear CCLers, 
I am trying to optimize and calculate the minimum geometry of a molecule in 
gaussian 98. How do I know that i have found that geometry in optimization 
and where do i search in the output file for imaginary frequencies? 

I have included here a cutout of my output wich shows negative low 
frequencies, but this is really a imaginary frequency? 
I am not a member of the CCL so please reply directly to my e-mail: 
pfernandes.-at-.cetem.gov.br 
N-N= 3.997713045864D+02 E-N=-1.603506616097D+03  KE= 3.445396433384D+02 
  Exact polarizability:  89.458  -0.107  39.829  -0.064   6.606 119.415 
 Approx polarizability: 150.116  -0.187  50.980  -0.029   2.556 165.619 
 Full mass-weighted force constant matrix: 
 Low frequencies ---   -8.0171   -0.0006    0.0004    0.0005   11.6757 
12.6686 
 Low frequencies ---   49.6615   95.7842  104.8786 
 Harmonic frequencies (cm**-1), IR intensities (KM/Mole), 
 Raman scattering activities (A**4/AMU), Raman depolarization ratios, 
 reduced masses (AMU), force constants (mDyne/A) and normal coordinates: 
                     1                      2                      3 
                    ?A                     ?A                     ?A 
 Frequencies --    49.5388                95.7622               104.8667 
 Red. masses --     2.5902                 1.9409                 3.9429 
 Frc consts  --     0.0037                 0.0105                 0.0255 
 IR Inten    --     0.0139                 0.0012                 0.4366 
 Raman Activ --     0.0000                 0.0000                 0.0000 
 Depolar     --     0.0000                 0.0000                 0.0000 
 Atom AN      X      Y      Z        X      Y      Z        X      Y      Z 
   1   6    -0.01   0.11   0.06     0.03   0.00  -0.04     0.00   0.09 
0.06 
   2   6     0.06   0.11   0.06    -0.02   0.00  -0.04     0.00  -0.12 
0.05 
   3   6     0.09   0.00   0.00    -0.04   0.00   0.00     0.00  -0.20 
0.04 

Here are the answers i received:
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I think you are done.  Starting with the first entry marked 

          1 
            ?A 
            49.5388 

you have a positive value.  SO this means that you have "no negative
frequencies" in you table.  The negative ones are listed first.  If you were
trying to optimize to a transition state, you would look for one negative
frequency here.  But just for an OPT run, this looks fine to me.
------------------------------------------------------------------------

That looks like a good clean minimum.  Youve found the right place to 
look at frequencies and the one negative low frequency that youv'e 
found is not of concern.  In genreal an negative frequency below 45cm-1 
is ok or if using DFT perhaps 60cm-1. 
To know that an optimization job has completed normaly look for a 
a section like this 
Maximum Force            0.000250     0.000450     YES 
 RMS     Force            0.000221     0.000300     YES 
 Maximum Displacement     0.000519     0.001800     YES 
 RMS     Displacement     0.000463     0.001200     YES 
 Predicted change in Energy=-2.225553D-07 
 Optimization completed. 
    -- Stationary point found. 
Alternatly you will probably fing the word GINC in the file.  Most 
sucesfull optimiztions will give you a quote at the end but this doesn't 
always indicate a sucesfull completion. 
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Hi! You are looking at the right place, but what you`ve got is not a 
transition state, it is some local or global minimum. For a TS, you should 
have a large negative number for the first frequency (instead of 49.5 cm-) 
here 

>                      1                      2                      3 
>                     ?A                     ?A                     ?A 
>  Frequencies --    49.5388                95.7622               104.8667 

By the way, you should have one and only one negative frequency. More than 
one (-) frequencies means this is a higher order saddle point, not a real 
transition state. 

The following numbers, I believe, correspond to the 6 non-vibrational 
degrees of freedom (3 for rotation + 3 for translation): 

>  Low frequencies ---   -8.0171   -0.0006    0.0004    0.0005   11.6757 
> 12.6686 

Ideally these should be zero of course, and they actually are pretty close 
to zero. 

Finally, even if you get 1 imaginary frequency, you should always check the 
motion of the atoms to make sure that it makes sense. That is, you may have 
ended up with a different TS than the one you want. 
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The first six low frequencies in the gaussian output (you will see that 
they are separated from the "other" low frequencies), correspond to the 
translation and rotation of the molecule as a whole, and they should 
ideally be zero. If you are looking for a transition state you need an 
imaginary frequency that corresponds to a normal vibrational mode. Once 
you have located a stationary point with one negative frequency, it is 
often useful to "animate" the vibrational modes (you can do it with a 
program like "molden") to check if the movement corresponds to the one 
you are looking for. For what I read in the output file you have sent, I 
would say that you have just located a minimum. 
------------------------------------------------------------------------

It is my understanding that the low frequencies listed are rotational 
frequencies that can be ignored.  The first "real" frequency would be the 
one labeled as "1", i.e., 49.5388. 

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