  Despite the popularity of scaled harmonic frequency calculations in
chemistry, sparse benchmarking is available to guide users on appropriate level
of theory and basis set choices (model chemistry). Here, we assess the
performance of over 600 model chemistries in scaled harmonic frequencies
calculations evaluating different scaling factors types.
  We can summarise our results into three main findings: (1) using model
chemistry-specific scaling factors optimised for three different frequency
regions (low (below 1,000cm-1), mid (1,000-2,000cm-1), and high (above
2,000cm-1)) results in better agreement between the scaled harmonic and
experimental frequencies compared to other choices; (2) larger basis sets and
more robust levels of theory generally lead to superior performance; however,
the particular model chemistry choice matters; and (3) outliers should be
expected in routine calculations.
  In terms of specific recommendations, overall, the highest accuracy model
chemistries are double-hybrid functionals with non-Pople augmented triple-zeta
basis sets, which can produce median frequency errors of down to 7.6cm-1
(DSD-PBEP86/def2-TZVPD) lying close to the error in the harmonic approximation.
Double-zeta basis sets should not be used with double-hybrid functionals as
there is no improvement compared to hybrid functionals. Note that the 6-311G*
and 6-311+G* bases perform like double-zeta basis sets for vibrational
frequencies.
  After scaling, all studied hybrid functionals with non-Pople triple-zeta
basis sets will produce median errors of less than 15cm-1, with the best result
of 9.9cm-1 with B97-1/def2-TZVPD. Appropriate matching of double-zeta basis
sets with hybrid functionals can produce high quality results. The B97-1,
TPSS0-D3(BJ) or wB97X-D hybrid functionals with 6-31G*, pc-1 or pcseg-1 are
recommended for fast routine calculations, all delivering median errors of
11-12cm-1.
