Somebody recommended the decrease of the symmetry, e.g., using C1, to eliminate it. I have just tried thiabendazole, which also can be considered as to be planar even in C1, using GAMESS with 6311++G(2d,2p) and b3lyp, and the approximate 55 cm1 imaginary frequency has been removed during HF vibrational anaysis with the warning meaning it is not a true stationary point even if the convergence criteria are met, thus HESSIAN analysis is not rigorious.
The obtained THz results with GAMESS exhibits two peaks in the range of 02.0 THz, assuming 1THZ =33.33 cm1.
MODE FREQ(CM**1) SYMMETRY RED. MASS IR INTENS.
1 0.026 A ... 0.000000
2 0.004 A ... 0.000000
3 0.034 A ... 0.000000
4 33.966 A ... 0.000687
5 50.002 A ... 0.010410
...
If the imaginary frequency has not been eliminated, it gives
MODE FREQ(CM**1) SYMMETRY RED. MASS IR INTENS.
1 55.684 A" ... 0.105512
2 1.891 A" ... 0.090056
3 0.114 A" ... 0.001076
4 0.052 A' ... 0.000036
5 0.034 A' ... 0.000001
6 0.814 A' ... 0.010790
7 1.182 A" ... 0.000087
8 66.596 A" ... 0.068779
...
1 2 3 4 5
FREQUENCY: 55.68 I 1.89 0.11 0.05 0.03
SYMMETRY: A" A" A" A' A'
REDUCED MASS: ...
IR INTENSITY: 0.10551 0.09006 0.00108 0.00004 0.00000
...
NWCHEM SCF and 631+G(d) frequency analysis gives
Normal Eigenvalue  Projected Infra Red Intensities
Mode [cm**1]  [atomic units] [(debye/angs)**2] [(KM/mol)] [arbitrary]
      
1 0.000  0.000331 0.008 0.323 0.196
2 0.000  0.002410 0.056 2.350 1.424
3 0.000  0.000866 0.020 0.844 0.512
4 0.000  0.000963 0.022 0.939 0.569
5 0.000  0.000105 0.002 0.102 0.062
6 0.000  0.000074 0.002 0.073 0.044
7 92.624  0.000461 0.011 0.449 0.272
8 99.156  0.000909 0.021 0.886 0.537
...
