The molecular parameters of the ground triplet state of cyanocarbene, (3)A" HCCN, have been determined in large-scale ab initio calculations using the coupled-cluster method, RCCSD(T), and basis sets of double-through-quintuple-zeta quality. The equilibrium structure of the molecule was found to be planar and bent, with the trans conformation of the HCCN chain and the parameters r(e)(HC) = 1.069 Angstrom, r,(CC) = 1.328 Angstrom, r(e)(CN) = 1.186 Angstrom, angle(e)(HCC) = 144.9degrees, and angle(e)(CCN) = 175.4degrees. The potential energy function for the HCC bending motion (nu(5) mode) was determined to be strongly anharmonic, with the barrier to linearity of 286 cm(-1). Influence of various electron-correlation effects on the shape of the HCC bending potential energy function is discussed. The rotation-bending energy levels of the HCCN and DCCN molecules were then calculated using a semirigid-bender Hamiltonian. For both molecules, the predicted patterns of rotational transitions in the excited nu(5) states agree favorably with the experimental data.