A new six-dimensional <i>ab initio</i> potential energy surface and rovibrational spectra for the CO–CO2 complex

New six-dimensional potential energy surfaces containing the stretching vibration of CO and the Q3 normal mode for the ν3 antisymmetric stretching vibration of CO2 were constructed for the CO-CO2 complex at the CCSD(T)-F12/AVTZ level. We obtained the effective vibrational interaction potentials by integration over the two intramolecular coordinates, and these potentials are found to exhibit a global T-shaped minimum (the C-bonded isomer) with the C atom in CO pointed toward CO2 and a local T-shaped minimum (the O-bonded isomer) but with the CO monomer flipped by 180°. The rovibrational energy levels and bound states were computed by employing the radial discrete variable representation/angular finite basis representation approach and the Lanczos algorithm. The band origin shifts of infrared spectra in the stretching range of CO are +5.022 and -3.589 cm-1 for the two isomers, which are in accordance with the measured data of +4.970 and -2.982 cm-1, respectively, and better than the values of +5.3/+3.6 and -0.6/-0.4 cm-1 obtained from previous theoretical studies. In addition, the wavefunctions of the intermolecular ground and lower excited vibrational states distribute around the T-shaped minima, and the obtained intermolecular vibrational frequencies for both isomers performed on the averaged potential energy surfaces are close to the available observed values. The spectroscopic constants fitted from the rovibrational levels exhibit that the CO-CO2 dimer is a near antisymmetric prolate top. The obtained pure rotational transition energies and the infrared fundamental and combination bands of the CO-CO2 dimer and the isotopomer 13CO-CO2 dimer well reproduce the experimental spectra.