The effect of relativistic interactions on the spectral characteristics of the ground state of carbon monoxide

Using high-precision nonempirical methods of modern quantum chemistry, the effect of the weak relativistic interactions on the potential energy and the permanent dipole moment of the ground electronic state of the CO molecule is studied. The relativistic energy is calculated by the following three optional methods: within the first-order perturbation theory using the Cowan–Griffin operator containing the sum of the mass-velocity and Darwin corrections, within the framework of the approximate Douglas–Kroll–Hess scalar Hamiltonian, and the most rigid “four-component” relativistic Dirac–Coulomb–Gaunt Hamiltonian. The relativistic correction obtained by different methods agrees within a few percents and equals about 55–60 cm$^{-1}$ in the region of an equilibrium internuclear distance of $R_e$ = 1.128 $\mathring{\mathrm{A}}$. The addition of the relativistic correction decreases the equilibrium bond length by about 0.0002 $\mathring{\mathrm{A}}$. The magnitude of the Lamb shift estimated by the semiempirical scaling of the one-electron Darwin's term does not exceed several inverse centimeters near $R_{e}$. The relativistic correction to the dipole moment function is in the range from -0.001 to +0.003 D, which does not exceed 1% of the nonrelativistic component of the dipole moment.