Any tips for getting rid of small imag. frequencies?

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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

  • Bert Forum:Admin, Forum:Mod, NWChemDeveloper, bureaucrat, sysop
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You may have to tighten the convergence of the optimization, but also of the SCF/DFT.

driver
 tight
end

scf
thresh 1.0d-8
end

dft
tolerances tight
grid fine
convergence energy 1.0d-8
end

Bert

Quote:Stigrs Mar 27th 6:18 am
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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