# Example: Rescan frequencies: NH3¶

Download Freq_NH3_Scan.run

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 were 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.

$ADFBIN/adf <<eor title NH3 analytic frequencies atoms N 0.0000 0.0000 0.0000 H 0.4729 0.8190 0.3821 H -0.9457 0.0000 0.3821 H 0.4729 -0.8190 0.3821 end Basis Type TZP Core Small End AnalyticalFreq End BeckeGrid Quality verygood End end input eor mv TAPE21 NH3_anl.t21$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
BeckeGrid
Quality verygood
End
end input
eor