A purely ab initio spectroscopic quality quartic force field for acetylene

The quartic force field of acetylene was determined using the $CCSD(T)$ method (coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected triple excitations) with a variety of one-particle basis sets of the atomic natural orbital, correlation consistent, and augmented correlation consistent types. The harmonic $πg$ bending frequency $ω4$ and the corresponding anharmonicity $ω4−ν4$ are both found to be extremely sensitive to the basis set used, in particular to the presence of a sufficient complement of diffuse functions. (Due to symmetry cancellation, the corresponding effect on the $πu$ mode, i.e., $ω5$ and $ω5−ν5,$ is much weaker.) Similar phenomena are observed more generally in bending modes for molecules that possess carbon–carbon multiple bonds. Tentative explanations are advanced. Our best computed quartic force field, which combines $CCSD(T)/[6s5p4d3f2g/4s3p2d1f]$ anharmonicities with a geometry and harmonic frequencies that additionally include inner-shell correlation effects, reproduces the observed fundamentals for HCCH, HCCD, DCCD, $H13CCH,$ and $H13C13CH$ with a mean absolute error of $1.3 cm−1,$ and the equilibrium rotational constant to four decimal places, without any empirical adjustment. Anharmonicity and quartic resonance constants are in excellent agreement with the recent determination of Temsamani and Herman [J. Chem. Phys. 103, 6371 (1995)], except for the vibrational $l$-doubling constant $R45,$ for which an adjustment to the computed force field is proposed.