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 pi(g) bending frequency omega(4) and the corresponding anharmonicity omega(4)-nu(4) 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 pi(u) mode, i.e., omega(5) and omega(5)-nu(5), 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, (HCCH)-C-13, and (HCCH)-C-13-C-13 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 R-45, for which an adjustment to the computed force field is proposed. (C) 1998 American Institute of Physics.