The quartic
potential energy surface of H_{2}O was computed using the
coupled-cluster singles, doubles, and perturbatively-applied triples [CCSD(T)]
level of theory. The cc-pVXZ, aug-cc-pVXZ, and aug-cc-pCVXZ (X = D-5) families
of basis sets were used for these computations. Harmonic frequencies, anharmonic
constants, and resonance constants were computed using second-order vibrational
perturbation theory (VPT2) including the k_{11,33} Darling-Dennison
resonance, and vibrational energy levels were predicted using these (harmonic, anharmonic,
and resonance) spectroscopic constants. The spectroscopic constants were fit
to experimental vibrational energy levels, and the standard deviation of the
fit was 3.7 cm^{-1}. The average absolute difference between the
theoretical harmonic frequencies and the fitted harmonic frequencies (|error|) was used
to gauge the accuracy of the computations. The (|error|) quantity for computed harmonic
frequencies and anharmonic constants at a particular basis set were compared to
that with the next smallest basis set by an average absolute difference (|D|) as a measure of convergence. Using the aug-cc-pVXZ
basis sets, the harmonic frequencies are converged to less than 3 cm^{-1}
at X = 4 (|error|_{QZ}
= 2.76 cm^{-1};|D_{5Z-QZ}| = 1.29 cm^{-1}).
The anharmonic constants converge more rapidly, and using the cc-pVXZ series of
basis sets, they are converged to less than 2 cm^{-1} at X = T (|error|_{TZ}
= 1.89 cm^{-1};|D_{QZ-TZ}| = 0.03 cm^{-1}).
Thus, a scheme was devised for a set of hybrid computations in which the
harmonic part of the potential was computed using CCSD(T)/aug-cc-pVQZ while the
more expensive cubic and quartic potential terms were computed using
CCSD(T)/cc-pVTZ, and the vibrational energy levels were predicted using this
hybrid potential energy surface. Using this aug-cc-pVQZ/cc-pVTZ hybrid model, |error| = 3.56 cm^{-1}
and 2.31 cm^{-1} for the harmonic frequencies and anharmonic constants,
respectively. Results for H_{2}CO, HFCO, HCO are also presented.