Six-dimensional potential energy surface and rovibrational energies of the HCCN radical in the ground electronic state

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 and 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 4 bending mode and to the quasilinear 5 bending mode are identified. It has been also found that v2 and v3 + (v4^1,V5^1)^0,0 are coupled in HCCN, while the mixing between v3 and (2v4^0,2v5^0)^0,0 is seen in DCCN.