Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the A–X emission spectrum

We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled-cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero-point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH-overtone states up to 23 063 cm−1. We also report a simulation of the ÖX̃ stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even-bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited à state. The potential for the à state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the à state. In addition, the Ã-state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis-switching arguments of a number of previously unassigned states in the SEP spectrum.