An internally contracted multireference coupled cluster treatment of the CO ground state in a wide range of internuclear distances

Abstract

The internally contracted multireference coupled cluster (ic-MRCC) method was applied for large-scale calculations of the potential energy curve and the permanent dipole moment function (DMF) for the ground X1Σ+ state of the CO molecule in the range of internuclear distances R from 0.7 to 3.0 Å, covering the energy range significantly higher than the last spectroscopically observed vibrational level v = 41. The ic-MRCCSD(T){4} approach, perturbatively accounting for contributions of triple excitation cluster amplitudes with up to four active indices, results in ab initio vibrational term values with a maximum deviation from their experimental counterparts not exceeding 10–15 cm−1 for v ∈ [0, 41]. The respective DMF obtained by the present ic-MRCC calculation agrees within ±0.005 D with its most accurate theoretical and semi-empirical counterparts in the interval R ∈ [0.8, 1.5] Å, but systematically diverges for R > 1.8 Å up to 0.1 D. It highlights the necessity for the revised intensity measurements between high-lying (v > 7) vibrational levels of the CO ground state since the observed discrepancy is unlikely to be attributed to the lack of higher excitations in the cluster expansion or the incompleteness of basis sets employed. The ic-MRCC method can be recommended for highly accurate calculations of electronic structure and properties of other small atmospheric molecules.