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 (RMSDs) from experiment are 0.04 pm for the equilibrium bond distances (re ), 1.03 cm −1
1.03 cm−1
for the harmonic frequencies (ωe) , 0.20 cm −1
0.20 cm−1
for the first anharmonicity constants (ωexe)
, 0.10 cm −1
0.10 cm−1
for the second anharmonicity constants (ωeye)
, and 0.001 cm −1
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
2.0 cm−1
for ω e
ωe
, 0.4 cm −1
0.4 cm−1
for ωexe, and 0.2 cm −1
0.2 cm−1
for ωe ye. We find that post-CCSD(T) contributions are essential to achieve such narrow confidence intervals for r e
re
and ω e
ωe
, but have little effect on ω e x e
ωexe
and α e
αe
, and virtually none on ω e y e
ωeye
. Higher-order connected triples T ˆ 3 −(T)
T̂3−(T)
improve the agreement with experiment for the hydride systems, but their inclusion (in the absence of T ˆ 4
T̂4
) tends to worsen the agreement with experiment for the nonhydride systems. Connected quadruple excitations T ˆ 4
T̂4
have significant and systematic effects on
r e
, 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
32 cm−1
), and universally increase ωex e (by up to 1 cm −1
1 cm−1
). Connected quintuple excitations T ˆ 5
T̂5
are spectroscopically significant for ω e
ωe
of the nonhydride systems, affecting ωe
by up to 4 cm −1
4 cm−1
. Diagonal Born–Oppenheimer corrections have systematic and spectroscopically significant effects on re
and ωe
of the hydride systems, universally increasing r e
re
by 0.01–0.06 pm and decreasing ω e
ωe
by 0.3–2.1 cm −1
0.3–2.1 cm−1
. Obtaining r e
re
and ω e
ωe
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 H2 O , 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.
Original language | English |
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Pages (from-to) | 144102 (1-17) |
Journal | Journal of Chemical Physics |
Volume | 133 |
Issue number | 14 |
DOIs | |
Publication status | Published - 11 Oct 2010 |