Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
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Vibrational frequency comparison for (Formaldehyde)

Frequency FAQs

Q. Why is there a difference between the 6-31G* and 6-31G** basis sets when the molecule contains no hydrogens? Shouldn't they be the same?
A. The raw calculated frequencies should be the same. But the two basis sets may have different vibrational scaling factors, which leads to different final vibrational frequencies.