Relativistic, quantum-electrodynamic and electron-correlation corrections to the interatomic potential of the CN radical

The dependence of the ground-state $(X^2\Sigma^+)$ potential energy of the CN molecule on the internuclear distance was determined using the single-reference coupled-cluster method. Contributions to the electronic energy arising from relativistic effects, including spin-orbit interaction and the Gaunt correction, as well as quantum electrodynamics corrections implemented through a generalized relativistic pseudopotential model, were evaluated. The incorporation of these factors resulted in good agreement between the calculated equilibrium bond distance $R_{\mathrm{e}}$ and harmonic vibrational frequency $\omega_{\mathrm{e}}$ with experimental data.