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Accurate spectroscopic constants and electrical properties of small moleculesare determined by means of W4 and post-W4 theories. For a set of 28 first- andsecond-row diatomic molecules for which very accurate experimentalspectroscopic constants are available, W4 theory affords near-spectroscopic orbetter predictions. Specifically, the root-mean-square deviations (RMSD) fromexperiment are 0.04 pm for the equilibrium bond distances (r_e), 1.03 cm^{-1}for the harmonic frequencies (\omega_e), 0.20 cm^{-1} for the firstanharmonicity constants (\omega_e x_e), 0.10 cm^{-1} for the secondanharmonicity constants (\omega_e y_e), and 0.001 cm^{-1} for thevibration-rotation coupling constants (\alpha_e). Higher-order connectedtriples, \hat{T}_3-(T), improve agreement with experiment for the hydridesystems, but their inclusion (in the absence of \hat{T}_4) tends to worsenagreement with experiment for the nonhydride systems. Connected quadrupleexcitations, \hat{T}_4, have significant and systematic effects on r_e,\omega_e, and \omega_e x_e, in particular they universally increase r_e (by upto 0.5 pm), universally reduce \omega_e (by up to 32 cm^{-1}), and universallyincrease \omega_e x_e (by up to 1 cm^{-1}). Connected quintuple excitations,\hat{T}_5, are spectroscopically significant for \omega_e of the nonhydridesystems, affecting \omega_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 BeOdiatomics, are also considered. The asymmetric stretch of ozone represents asevere challenge to W4 theory, in particular the connected quadruplecontribution converges very slowly with the basis set size. Finally, theimportance of post-CCSD(T) correlation effects for electrical properties,namely dipole moments (\mu), polarizabilities (\alpha), and firsthyperpolarizabilities (\beta) is evaluated.

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