NH₃: rescan frequencies

Sample directory: adf/Freq_NH3_Scan/

Sometimes spurious imaginary frequencies are calculated where one would expect a very low (nearly zero) frequency. Most frequently this happens when there is a barrier-free rotation of, for example, methyl groups. The SCANFREQ keyword allows one to rescan calculated frequencies in order to find out if they wre calculated accurately.

In this example analytical frequencies are calculated. Next recalculation of certain NH3 frequencies are performed by scanning along normal modes from a restart file. In this calculation the frequencies are calculated numerically with finite displacements using symmetry.

$ADFBIN/adf <<eor
title NH3 analytic frequencies 
atoms
   N               0.0000    0.0000    0.0000
   H               0.4729    0.8190    0.3821
   H              -0.9457    0.0000    0.3821
   H               0.4729   -0.8190    0.3821
end
Basis
 Type TZP
 Core Small
End
AnalyticalFreq
End
integration  5.0
end input
eor

mv TAPE21 NH3_anl.t21

$ADFBIN/adf <<eor
title Re-calculate NH3 frequencies by scanning along normal modes from a restart file
atoms
   N               0.0000    0.0000    0.0000
   H               0.4729    0.8190    0.3821
   H              -0.9457    0.0000    0.3821
   H               0.4729   -0.8190    0.3821
end
Fragments
  N t21.N
  H t21.H
End
ScanFreq 0 4000
Restart NH3_anl.t21
integration  5.0
end input
eor