  We show that the DCSD (distinguishable clusters with all singles and doubles)
correlation method permits the calculation of vibrational spectra at
near-CCSD(T) quality but at no more than CCSD cost, and with comparatively
inexpensive analytical gradients. For systems dominated by a single reference
configuration, even MP2.5 is a viable alternative, at MP3 cost. MP2.5
performance for vibrational frequencies is comparable to double hybrids such as
DSD-PBEP86-D3BJ, but without resorting to empirical parameters. DCSD is also
quite suitable for computing zero-point vibrational energies in computational
thermochemistry.
