CO line intensities: Towards subpercent accuracy of intensities of all bands

Accurate predictions of the carbon monoxide dipole moment as a function of geometry provide a challenge to theory not least because lower-level theories such as Hartree–Fock give dipoles with the wrong sign at equilibrium making reliable calculations of even the intensity of pure rotational transitions difficult. Intensities of CO lines are calculated using higher level theory based on multi-reference configuration interaction (MRCI). Tests using an increased complete active space (CAS) compared to previous high accuracy studies (Balashov et al., 2023), show that while increasing the σ orbital space from 7 to 8 orbitals produces poor (unbalanced) results, increasing the π space from 2 to 3 gives an ab initio model with reproduced CO pure rotational line intensities at the sub-percent level for the first time. This dipole function gives line intensities of all observed vibrational bands of CO from 0–0 to 7–0 with the sub-percent or experimental accuracy (whichever is the larger). Other possible improvement to the model such as use of larger (7 zeta) basis sets and relativistic corrections are considered. The permanent dipole of CO at its equilibrium geometry is estimated to be − 0 . 1236 D with an uncertainty of 0.0003 D. • A high ab initio dipole moment curve for CO is presented. • This curve gives excellent predictions for all know rotation vibration bands of CO including the pure rotational band. • A value for the equilibrium dipole moment of CO of -0.1236(3) D is deduced.