Ab initio global potential, dipole, adiabatic, and relativistic correctionsurfaces for the HCN-HNC system

Ab initio semiglobal potential energy and dipole moment hypersurfaces for t he isomerising HCN-HNC system are computed, using a grid of 242 points, pri ncipally at the all-electron cc-pCVQZ CCSD(T) level. Several potential ener gy hypersurfaces (PES) are presented including one which simultaneously fit s 1527 points from earlier ab initio, smaller basis CCSD(T) calculations of Bowman [J. Chem. Phys. 99, 308 (1993)]. The resulting potential is then mo rphed with 17 aug-cc-pCVQZ CCSD(T) points calculated at HNC geometries to i mprove the representation of the HNC part of the surface. The PES is furthe r adjusted to coincide with three ab initio points calculated, at the cc-pC V5Z CCSD(T) level, at the critical points of the system. The final PES incl udes relativistic and adiabatic corrections. Vibrational band origins for H CN 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 trans itions for HCN have been calculated with the new dipole surface, giving gen erally very good agreement with experiment. The rotational levels of ground and vibrationally excited states are reproduced to high accuracy. (C) 2001 American Institute of Physics.