We report large-scale quantum mechanical calculations for the HCCN radical in its ground electronic state. A six-dimensional potential energy surface based on MR-ACPF/cc-pVQZ ab initio energy points is developed and adjusted to reproduce experimental findings for nu(1)(5) and nu(1) of HCCN. Rovibrational energy levels of HCCN and DCCN are computed for total rotational angular momentum J = 0-4 by making use of combined (functional + point wise) coordinate representations together with contraction schemes resulting from several diagonalization/truncation steps. The classical barrier to linearity is determined to be 287 cm(-1). Spectroscopic parameters are calculated for low lying states and compared with available experimental data. Energy patterns attributed to the nu(4) bending mode and to the quasilinear nu(5) bending mode are identified. It has been also found that nu(2) and nu(3) + (nu(1)(4),nu(1)(5))(0,0) are coupled in HCCN, while the mixing between nu(3) and (2 nu(0)(4), 2 nu(0)(5))(0,0) is seen in DCCN.