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Abstract: 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-CCSDT correlation effects for electrical properties,namely dipole moments \mu, polarizabilities \alpha, and firsthyperpolarizabilities \beta is evaluated.



Author: Amir Karton, Jan M.L. Martin

Source: https://arxiv.org/



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