Frequency and Zero-Point Vibrational Energy Scale Factors for Double-Hybrid Density Functionals (and Other Selected Methods): Can Anharmonic Force Fields Be Avoided?

We
have obtained uniform frequency scaling factors λharm (for harmonic frequencies), λfund (for fundamentals),
and λZPVE (for zero-point vibrational energies (ZPVEs))
for the Weigend–Ahlrichs and other selected basis sets for
MP2, SCS-MP2, and a variety of DFT functionals including double hybrids.
For selected levels of theory, we have also obtained scaling factors
for true anharmonic fundamentals and ZPVEs obtained from quartic force
fields. For harmonic frequencies, the double hybrids B2PLYP, B2GP-PLYP,
and DSD-PBEP86 clearly yield the best performance at RMSD = 10–12
cm–1 for def2-TZVP and larger basis sets, compared
to 5 cm–1 at the CCSD­(T) basis set limit. For ZPVEs,
again, the double hybrids are the best performers, reaching root-mean-square
deviations (RMSDs) as low as 0.05 kcal/mol, but even mainstream functionals
like B3LYP can get down to 0.10 kcal/mol. Explicitly anharmonic ZPVEs
only are marginally more accurate. For fundamentals, however, simple
uniform scaling is clearly inadequate.