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 6-311++G(2d,2p) and b3lyp, and the approximate 55 cm-1 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 0-2.0 THz, assuming 1THZ =33.33 cm-1.
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 6-31+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
...
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