Ab initio global potential, dipole, adiabatic, and relativistic correction surfaces for the HCN–HNC system

Ab initio semiglobal potential energy and dipole moment hypersurfaces for the isomerising HCN–HNC system are computed, using a grid of 242 points, principally at the all-electron cc-pCVQZ CCSD(T) level. Several potential energy hypersurfaces (PES) are presented including one which simultaneously fits 1527 points from earlier ab initio, smaller basis CCSD(T) calculations of Bowman et al. [J. Chem. Phys. 99, 308 (1993)]. The resulting potential is then morphed with 17 aug-cc-pCVQZ CCSD(T) points calculated at HNC geometries to improve the representation of the HNC part of the surface. The PES is further adjusted to coincide with three ab initio points calculated, at the cc-pCV5Z CCSD(T) level, at the critical points of the system. The final PES includes relativistic and adiabatic corrections. Vibrational band origins for HCN and HNC with energy up to 12 400 cm⁻¹ above the HCN zero-point energy are calculated variationally with the new surfaces. Band transition dipoles for the fundamentals of HCN and HNC, and a few overtone and hot band transitions for HCN have been calculated with the new dipole surface, giving generally very good agreement with experiment. The rotational levels of ground and vibrationally excited states are reproduced to high accuracy.