ROTATION VIBRATION ENERGIES FOR THE HO2 MOLECULE

Walch and Duchovic [J. Chem. Phys.94, 7068-7075 (1991)] recently carried out complete active space SCF and externally contracted configuration interaction (CASSCF-CCI) ab initio calculations to determine the potential energy surface of the X ̃2A″ electronic ground state of the HO2 radical over a wide range of nuclear geometries. By fitting to 192 of their ab initio points, covering energies up to 12 000 cm-1 above the energy of the equilibrium geometry, we determine an analytical expression for the potential energy function in terms of the bond lengths and bond angle. The fitted potential gives Re(OH) = 0.971 A ̊ (0.971 A ̊), Re(OO) = 1.330 A ̊ (1.331 A ̊), and αe = 104.3° (104.3°), where the experimental values are in parentheses. We use the analytical ab initio potential function with the Morse Oscillator Rigid Bender Internal Dynamics (MORBID) Hamiltonian [J. Mol. Spectrosc.128, 478-501 (1988); J. Chem. Soc. Faraday Trans. 284, 1315-1340 (1988)] in a calculation of vibrational energies. We adjust four parameters in the potential in a fitting to experimental vibrational energies, and then calculate the energies of N = 0 and 1 levels up to 8000 cm-1 above the zero-point level using the MORBID approach. We have also used the Discrete Variable Representation and Distributed Gaussian Basis (DVR-DGB) approach to calculate the energies of the N = 0 levels for comparison. The aim of this work is to obtain a zeroth-order set of vibrational states (with their associated rotational constants) in the region between 7000 and 8000 cm-1 where there will be interaction with the low-lying A ̃2A′ state for which T0 = 7029 cm-1. © 1992.