On the ground and some low-lying excited states of ScB: A multiconfigurational study

The electronic structure of a series of low-lying excited triplet and quintet states of scandium boride (ScB) was examined using multireference configuration interaction (including Davidson's correction for quadruple excitations) and single-reference coupled cluster (CC) methods with averaged natural orbital (ANO) basis sets. The CC approach was used only for the lowest quintet state. The authors have analyzed eight low-lying triplets -3 (2), +3, Π3 (3), and Δ3 (2) dissociating to Sc (D2) B (P2) atoms and eight low-lying quintet states -5, +5, Π5 (2), 5, and Δ5 (3) dissociating to Sc (F4) B (P2) atoms. They report the potential energy curves and spectroscopic parameters of ScB obtained with the multireference configuration interaction (MRCI) technique including all singly and doubly excited configurations obtained with the ANO-S basis set. For the two lowest states they obtained also improved ANO-L spectroscopic constants, dipole and quadrupole moments as well as scalar relativistic effects based on the Douglas-Kroll-Hess Hamiltonian. They provide the analysis of the bonding based on Mulliken populations and occupation numbers. Since the two lowest states, -3 and -5, lie energetically very close, their principal goal was to resolve the nature of the ground state of ScB. Their nonrelativistic MRCI(Q) (including Davidson correction) results indicate that the quintet is more stable than the triplet by about 800 cm-1. Inclusion of scalar relativistic effects reduces this difference to about 240 cm-1. The dissociation energies for -5 ScB range from 3.20 to 3.30 eV while those for the -3 range from 1.70 to 1.80 eV.