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.