Calculations of low-lying vibrational states of cis- and trans-HOCO

We report variational calculations of low-lying vibrational states of non-rotating cis-HOCO and trans-HOCO, using the code 'MULTIMODE' and a realistic six degree-of-freedom potential surface. The results of these variational calculations are compared with previous five degree-of-freedom calculations [D.H. Zhang and J.Z.H. Zhang, J. Chem. Phys. 103, 6512 (1995)] in which the full six degree-of freedom potential is averaged over the 'spectator' CO stretch. This approximation is found to work better for cis-HOCO than trans-HOCO, and a simple explanation for this is presented. State-dependent rotation constants are calculated from exact rotation/vibration calculations for J =1 for low-lying states of cis- and trans-HOCO. (C) 1999 Elsevier Science B.V. All rights reserved.