Anharmonic Vibrational
Frequency Calculations Are
Not Worthwhile for Small Basis Sets
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Abstract
Anharmonic calculations using vibrational perturbation
theory are
known to provide near-spectroscopic accuracy when combined with high-level <i>ab initio</i> potential energy functions. However, performance
with economical, popular electronic structure methods is less well
characterized. We compare the accuracy of harmonic and anharmonic
predictions from Hartree–Fock, second-order perturbation, and
density functional theories combined with 6-31G(d) and 6-31+G(d,p)
basis sets. As expected, anharmonic frequencies are closer than harmonic
frequencies to experimental fundamentals. However, common practice
is to correct harmonic predictions using multiplicative scaling. The
surprising conclusion is that scaled anharmonic calculations are no
more accurate than scaled harmonic calculations for the basis sets
we used. The data used are from the Computational Chemistry Comparison
and Benchmark Database (CCCBDB), maintained by the National Institute
of Standards and Technology, which includes more than 3939 independent
vibrations for 358 molecules