The low-lying states of the scandium dimer.

A systematic investigation of low-lying states of Sc(2) using multireference perturbation theory (NEVPT2 and NEVPT3) indicates that the ground state of this system is (5)Sigma(u) (-) with r(e)=2.611 A, omega(e)=241.8 cm(-1), and D(e)=1.78 eV. This state is closely followed by other low-lying states of Sc(2): (3)Sigma(u) (-), (5)Delta(u), (3)Pi(g), (1)Pi(g), and (1)Sigma(u) (-). Our energy ordering of the (5)Sigma(u) (-) and (3)Sigma(u) (-) states confirms the recent MRCI results of Kalemos et al. [J. Chem. Phys. 132, 024309 (2010)] and is at variance with the earlier diffusion Monte Carlo predictions of Matxain et al. [J. Chem. Phys. 128, 194315 (2008)]. An excellent agreement between the second- and third-order NEVPT results and between the computed and experimental values of omega(e) (241.8 versus 238.9 cm(-1)) for the (5)Sigma(u) (-) state suggests high accuracy of our predictions.