Dear NWChem community,
When optimizing "floppy" molecules I often end up with a small negative frequency of about -20 cm-1 after projection of translations and rotations. Here is an example calculated at the B3LYP-D3/6-31+G* level of theory:
----------------------------------------------------------------------------
Normal Eigenvalue || Projected Infra Red Intensities
Mode [cm**-1] || [atomic units] [(debye/angs)**2] [(KM/mol)] [arbitrary]
------ ---------- || -------------- ----------------- ---------- -----------
1 -22.059 || 0.004045 0.093 3.943 1.105
2 -0.000 || 0.001142 0.026 1.113 0.312
3 -0.000 || 0.000296 0.007 0.288 0.081
4 -0.000 || 0.000091 0.002 0.089 0.025
5 0.000 || 0.000651 0.015 0.635 0.178
6 0.000 || 0.000741 0.017 0.722 0.202
7 0.000 || 0.000650 0.015 0.634 0.178
8 55.136 || 0.000230 0.005 0.224 0.063
9 77.074 || 0.000380 0.009 0.370 0.104
10 98.087 || 0.001099 0.025 1.072 0.300
11 110.271 || 0.002207 0.051 2.152 0.603
12 141.968 || 0.000884 0.020 0.862 0.241
...
Before projection the frequencies look like this:
----------------------------------------------------------------------------
Normal Eigenvalue || Infra Red Intensities
Mode [cm**-1] || [atomic units] [(debye/angs)**2] [(KM/mol)] [arbitrary]
------ ---------- || -------------- ----------------- ---------- -----------
1 -31.003 || 0.003451 0.080 3.364 0.943
2 -16.381 || 0.000803 0.019 0.783 0.219
3 -8.186 || 0.000064 0.001 0.062 0.017
4 8.536 || 0.000643 0.015 0.627 0.176
5 13.740 || 0.001031 0.024 1.005 0.282
6 14.686 || 0.000315 0.007 0.307 0.086
7 26.899 || 0.001201 0.028 1.171 0.328
8 54.709 || 0.000193 0.004 0.189 0.053
9 76.931 || 0.000378 0.009 0.369 0.103
10 98.600 || 0.001223 0.028 1.192 0.334
11 110.978 || 0.002085 0.048 2.033 0.570
12 142.041 || 0.000882 0.020 0.860 0.241
...
I have tried to start from the geometries printed after stepping along mode 1 without success. I have also tried to tighten the geometry convergence criteria - still no success.
Any suggestions on what to do is greatly appreciated!
Regards,
stigrs
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