Theoretical study of electronic structures of UNC and UCN

[Display omitted] •The lower quartet states are mainly arising from 5f37s2 configuration.•The fourth excited 6Δ states, which are lying within less than 0.1 eV are nearly degenerate.•4I states with splitting at 0, 0.469, 0.918 and 1.373 eV are dominated by the 4I spin-free state.•spin-orbit energy levels of UCN reveals that the successive levels are placed close to each other. All-electron relativistic ab initio calculations have been performed to calculate the electronic structures of UNC and UCN. The spin-free electronic states have been calculated at multireference complete active space second-order perturbation (CASPT2) theory level. The spin-orbit states have been computed using Douglas-Kroll type of atomic mean-field integral approach. The ground state geometries and also a manifold of low-lying spin-free states (blow 1.8 eV) and spin-orbit states (below 2.1 eV) have been calculated for both the systems. The ground states of both UNC and UCN are found to be linear with a spectral term of 4H(Λ = 5, 5fσ5fδ5fφ7s2) and 4I(Λ = 6, 5fл5fδ5fφ7s2), respectively. The strong electron correlation and also the strong configuration mixing in case of spin-free and the spin-orbit states lead to the complicated electronic structures of the excited states in both the systems.