High-level ab initio predictions for the ionisation energy, bond dissociation energies and heats of formation of zirconium oxide and its cation (ZrO/ZrO⁺)

The ionisation energy (IE) of ZrO, the bond dissociation energies (D₀s) and the heats of formation at 0 K (ΔH^○f^0K ) and 298 K (ΔH^○f ^298K ) for ZrO/ZrO⁺ are predicted by the coupled cluster methods utilizing upto single, double, triple and quadruple excitations and complete basis set limit approximation (CCSDTQ/CBS). The CCSDTQ/CBS approach also includes the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic and spin-orbit coupling corrections while the scalar relativistic contribution is handled by employing pseudopotential basis sets. The present calculations yield IE(ZrO) = 6.794 eV and D₀(Zr⁺–O) − D₀(Zr–O) = −0.193 eV which are in good agreement with the respective experimental values of 6.81272(10) eV and −0.1788(1) eV determined in a two-colour laser-pulsed field ionisation-photoelectron study. The CCSD(T) and multireference configuration interaction (MRCI) methods with CV correlations included give equally remarkable predictions of the harmonic frequencies and the bond lengths for ZrO/ZrO⁺. This study together with the previous investigations has demonstrated that, with pseudopotential basis sets, the CCSDTQ/CBS protocol can be readily extended to investigate 4d-transition metal-containing diatomic molecules to yield comparable accuracy (±20 meV) to that achieved in the IE and D₀ predictions of 3d-transition metal-containing species.