Example: Rescan frequencies: NH3¶
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