Subject: One negative eigenvalue - No imaginary frequency!!??

From: csilval <csilval!at!uvigo.es>
Date: Tue, 24 Aug 2004 16:29:51 -0500

Hi CCLers!

I have got a weird -well, to be honest, a couple of them- output files
> from g03 where I try to locate the TS of a nucleophilic attack. After
convergence a frequency job is performed. It results in one negative
eigenvalue but no imaginary frequency associated to it. Actually it
seems like there is one imaginary frequency, but gaussian is treating it
as a traslation/rotation. The lowest (but positive)  mode from the
vibrational analysis is likely to be the nucleophilic attack but I
cannot get it to be imaginary.

I have tried several basis sets and several levels of calculation (HF,
B3LYP, MP2) with the very same luck. Is there anything I can do to
locate the "real" TS.

Also I am enclosing here the very first modes of one of the structures
in case it helps to understand what is going on:

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Full mass-weighted force constant matrix:
 Low frequencies ---  -31.0607    0.0014    0.0024    0.0025   22.5484  
25.9445
 Low frequencies ---   42.3766   64.9775  107.1348
 Diagonal vibrational polarizability:
      624.6067824     849.1987473     446.1087874
 Harmonic frequencies (cm**-1), IR intensities (KM/Mole), Raman
scattering
 activities (A**4/AMU), depolarization ratios for plane and unpolarized
 incident light, reduced masses (AMU), force constants (mDyne/A),
 and normal coordinates:
                     1                      2                      3
                     A                      A                      A
 Frequencies --    18.6726                63.9044               105.9740
 Red. masses --     5.4210                 4.4793                 3.1978
 Frc consts  --     0.0011                 0.0108                 0.0212
 IR Inten    --    21.2490                13.5758                 6.6021
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Thanks a lot!