Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 !van der Waals complexes

The authors present a new five-dimensional potential energy surface for H-2-CO2 including the Q(3) normal mode for the nu(3) antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm(-1). The rovibrational energy levels for four species of H-2-CO2 (paraH(2)-, orthoH(2)-, paraD(2)-, and orthoD(2)-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu(3) region of CO2 is -0.113 cm(-1) for paraH(2)-CO2 and -0.099 cm(-1) for orthoH(2)-CO2, which agrees well with the observed values of -0.198 and -0.096 cm(-1). The calculated rovibrational spectra for H-2-CO2 are consistent with the available experimental spectra. For D-2-CO2, it is predicted that only a-type transitions occur for paraD(2)-CO2, while both a-type and b-type transitions are significant for orthoD(2)-CO2. (c) 2007 American Institute of Physics.