The dipole moment of carbon monoxide

The dipole moment of CO has been calculated with many-body perturbation theory (MBPT) and coupled cluster (CC) methods using basis sets which have been optimized at the MBPT-2 level. It is demonstrated that triple excitations as well as g-type functions in the basis set are crucial to obtain satisfactory agreement with experiment. The most reliable prediction (0.125 D) is obtained at the CCSD(T) (coupled cluster including all single, double, and connected triple excitations, perturbatively) level of theory using a 10s9p4d2f1g basis set (160 basis functions). This result is in excellent agreement with the experimental value of 0.122 D.