Performance of W4 theory for spectroscopic constants and electrical properties of small molecules

Abstract

Accurate spectroscopic constants and electrical properties of small molecules are determined by means of W4 and post-W4 theories. For a set of 28 first- and second-row diatomic molecules for which very accurate experimental spectroscopic constants are available, W4 theory affords near-spectroscopic or better predictions. Specifically, the root-mean-square deviations (RMSD) from experiment are 0.04 pm for the equilibrium bond distances (r_e), 1.03 cm^{-1} for the harmonic frequencies (ω_e), 0.20 cm^{-1} for the first anharmonicity constants (ω_e x_e), 0.10 cm^{-1} for the second anharmonicity constants (ω_e y_e), and 0.001 cm^{-1} for the vibration-rotation coupling constants (α_e). Higher-order connected triples, \hat{T}_3-(T), improve agreement with experiment for the hydride systems, but their inclusion (in the absence of \hat{T}_4) tends to worsen agreement with experiment for the nonhydride systems. Connected quadruple excitations, \hat{T}_4, have significant and systematic effects on r_e, ω_e, and ω_e x_e, in particular they universally increase r_e (by up to 0.5 pm), universally reduce ω_e (by up to 32 cm^{-1}), and universally increase ω_e x_e (by up to 1 cm^{-1}). Connected quintuple excitations, \hat{T}_5, are spectroscopically significant for ω_e of the nonhydride systems, affecting ω_e by up to 4 cm^{-1}. The triatomic molecules H_2O, CO_2, and O_3, as well as the pathologically multireference BN and BeO diatomics, are also considered. The asymmetric stretch of ozone represents a severe challenge to W4 theory, in particular the connected quadruple contribution converges very slowly with the basis set size. Finally, the importance of post-CCSD(T) correlation effects for electrical properties, namely dipole moments (μ), polarizabilities (α), and first hyperpolarizabilities (β) is evaluated.

arXiv commentJournal of Chemical Physics, in press

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Cited by in corpus (6)

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• High-level ab initio quartic force fields and spectroscopic characterization of C${}_2$N${}^{-}$