Analytic Morse/long-range potential energy surfaces and predicted infrared spectra for CO2-H2.

Five-dimensional ab initio potential energy surfaces (PESs) for CO(2)-H(2) that explicitly incorporate dependence on the Q(3) asymmetric-stretch normal-mode coordinate of the CO(2) monomer and are parametrically dependent on its Q(1) symmetric-stretch coordinate have been calculated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for v(3)(CO(2)) = 0, and 1 to the Morse/long-range potential function form. These fits to 23,113 points have root-mean-square (rms) deviations of 0.143 and 0.136 cm(-1), and require only 167 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data: for infrared transitions of para- and ortho-H(2)-CO(2), the rms discrepancies are only 0.004 and 0.005 cm(-1), respectively. The calculated infrared band origin shifts associated with the nu(3) fundamental of CO(2) are -0.179 and -0.092 cm(-1) for para-H(2)-CO(2) and ortho-H(2)-CO(2), in good agreement with the (extrapolated) experimental values of -0.198 and -0.096 cm(-1).