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

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 (RMSDs) 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_ex_e), 0.10 cm^-1 for the second anharmonicity constants (ω_ey_e), and 0.001 cm^-1 for the vibration-rotation coupling constants (α_e). These RMSDs imply 95% confidence intervals of about 0.1 pm for r_e, 2.0 cm^-1 for ω_e, 0.4 cm^-1 for ω_ex_e, and 0.2 cm^-1 for ω_ey_e. We find that post-CCSD(T) contributions are essential to achieve such narrow confidence intervals for r_e and ω_e, but have little effect on ω_ex_e and α_e, and virtually none on ω_ey_e. Higher-order connected triples T̂3-(T) improve the agreement with experiment for the hydride systems, but their inclusion (in the absence of T̂4) tends to worsen the agreement with experiment for the nonhydride systems. Connected quadruple excitations T̂4 have significant and systematic effects on r_e, ω_e, and ω_ex_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_ex_e (by up to 1 cm^-1). Connected quintuple excitations T̂5 are spectroscopically significant for ω_e of the nonhydride systems, affecting ω_e by up to 4 cm^-1. Diagonal Born-Oppenheimer corrections have systematic and spectroscopically significant effects on r_e and ω_e of the hydride systems, universally increasing r_e by 0.01-0.06 pm and decreasing ω_e by 0.3-2.1 cm^-1. Obtaining r_e and ω_e of the pathologically multireference BN and BeO systems with near-spectroscopic accuracy requires large basis sets in the core-valence CCSD(T) step and augmented basis sets in the valence post-CCSD(T) steps in W4 theory. The triatomic molecules H2O, CO2, and O3 are also considered. The equilibrium geometries and harmonic frequencies (with the exception of the asymmetric stretch of O3) are obtained with near-spectroscopic accuracy at the W4 level. 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.