Revisiting the nature of the ZnO ground state: Influence of spin–orbit coupling

Relativistic calculations of the low-lying electronic states of the ZnO molecule are performed for the Λ–Σ states, 1Σ+, 1Π, 1Δ, 3Π and 3Σ−, at the CCSD(T) or MRCI level, using scalar relativistic energy-consistent pseudopotentials, and the EPCISO method for spin–orbit CI coupling. The ZnO ground state is assigned to 0+ symmetry and has 1Σ+ character around the equilibrium region. The spectroscopic constants (re, ωe) of the 0+ ground state are in good agreement with experimental results. Interpenetration of the vibrational levels of the two lowest 0+ states is also shown.