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III.G.7. (XIII.B.3.) (XIII.D.5.)

Compare vibrational frequencies for two calculations for C2H+ (Ethynyl cation)

A = HF/6-31G*

B = MP2/6-31G*

scale factors=0.8985, 0.943
  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   3128 3197 -69.8 0.978   1.130 1.175 -0.045 0.961   197.15 258.41 -61.26 0.763
2 Σ Σ   1629 2003 -374.2 0.813   6.494 5.315 1.179 1.222   44.53 3.93 40.60 11.331
3 Π Π   840 760 80.3 1.106   1.323 1.331 -0.008 0.994   14.65 24.14 -9.49 0.607
4 Π Π   495 742 -246.9 0.667   1.323 1.331 -0.008 0.994   60.92 44.60 16.33 1.366
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.