An ab lnitio Calculation of the Low Rotation-Vibration Energies of the CO Dimer

On the basis of a previously published ab initio potential surface for the CO dimer [A. van der Pol, A. van der Avoird and P. E. S. Wormer, J. Chem. Phys.92, 7498-7504 (1990); the potential assumes fixed CO bondlengths] we have calculated the J < 3 rotation-vibration energies below 25 cm-1. This should be suitable for interpreting the absorption spectrum of CO dimer cooled to about 10 K. We have made uncoupled one- and three-dimensional calculations of the energies, as well as two different four-dimensional calculations. The first four-dimensional calculation uses an adiabatic separation of the intermolecular coordinate R, and the second uses the full close-coupling method. It is shown that because of the strong couplings on this surface there are differences between the results obtained from these calculations. However, even using the close-coupling results we cannot assign the five published experimental frequencies of CO dimer at 10K [P. A. Vanden Bout, J. M. Steed, L. S. Bernstein, and W. Klemperer, Astrophys. J.234, 503-505 (1979)]. © 1993 Academic Press. All rights reserved