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Compare vibrational frequencies for two calculations for HCN+ (hydrogen cyanide cation)


A = HF/6-31G*
B = MP2/6-31G*

scale factors=0.8985, 0.943 -3.068 4.279
  symmetry   frequency (cm-1)   reduced mass (amu)   IR Intensity (km mol-1)
mode number A B   A B diff. ratio   A B diff. ratio   A B diff. ratio
1 Σ Σ   3073 3743 -669.8 0.821   1.170 4.238 0.276   261.48 1.02 256.683
2 Σ Σ   1872 2790 -918.0 0.671   5.529 1.250 4.423   47.81 335.60 0.142
3 Π Π   784 910 -126.0 0.862   1.337 1.339 -0.001 0.999   51.94 36.09 1.439
4 Π Π   725 417 308.0 1.738   1.337 1.339 -0.001 0.999   4.51 0.16 27.402
scaled by     0.8985 0.943

See section III.C.1 List or set vibrational scaling factors to change the scale factors used here.
See section III.C.2 Calculate a vibrational scaling factor for a given set of molecules to determine the least squares best scaling factor.