Example: Rescan frequencies: NH3¶

#! /bin/sh

# 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 VibrationalAnalysis 
# task (using ModeRefinement) 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.

AMS_JOBNAME=NH3 $AMSBIN/ams <<eor
System
  Symmetrize Yes
  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
end

Task SinglePoint

Properties
  NormalModes Yes
End

Engine ADF
  title NH3 analytic frequencies
  beckegrid quality=Good
  basis
    type TZP
    core Small
  end
EndEngine
eor

AMS_JOBNAME=ScanFreq $AMSBIN/ams <<eor
LoadSystem 
  File NH3.results/adf.rkf
End
LoadEngine NH3.results/adf.rkf

Task VibrationalAnalysis

VibrationalAnalysis
    Type ModeRefinement
    NormalModes
       ModeFile NH3.results/adf.rkf
       ModeSelect
          FreqRange 0 4000
       End
    End
End
eor