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IV.C.1. (XIII.D.1.)

Compare vibrational frequencies in CCCBDB for C4H2 (Diacetylene)

PBEPBE/6-31G*

Frequency in cm-1
  Calculated   Experimental
Mode Number Symmetry Frequency diff Symmetry Frequency Comment
1 Σg 3376 83 Σg+ 3293  
2 Σg 2202 18 Σg+ 2184  
3 Σg 906 32 Σg+ 874  
4 Σu 3379 50 Σu+ 3329  
5 Σu 2028 8 Σu+ 2020  
6 Πg 644 17 Πg 627  
7 Πg 545 63 Πg 482  
8 Πu 504 -126 Πu 630  
9 Πu 256 25 Πu 231  
The calculated vibrational frequencies were scaled by 0.9857

See section XIII.C.3 List or set vibrational scaling factors to change the scale factors used here.
See section III.B.3.c Calculate a vibrational scaling factor for a set of molecules to determine the least squares best scaling factor.
See section III.B.3.b List of vibrational scaling factors and their uncertainties for information on where the above scaling factor is from.