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 (re), 1.03 cm-1 for the harmonic frequencies (ωe), 0.20 cm-1 for the first anharmonicity constants (ωe xe), 0.10 cm-1 for the second anharmonicity constants (ωe ye), and 0.001 cm-1 for the vibration-rotation coupling constants (αe). Higher-order connected triples, T3-(T), improve agreement with experiment for the hydride systems, but their inclusion (in the absence of T4) tends to worsen agreement with experiment for the nonhydride systems. Connected quadruple excitations, T4, have significant and systematic effects on re, ωe, and ωe xe, in particular they universally increase re (by up to 0.5 pm), universally reduce ωe (by up to 32 cm-1), and universally increase ωe xe (by up to 1 cm-1). Connected quintuple excitations, T5, are spectroscopically significant for ωe of the nonhydride systems, affecting ωe by up to 4 cm-1. The triatomic molecules H2O, CO2, and O3, 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.
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