Trends in low-lying electronic states of XH2 (XÂ =Â Li, Na, K)

Relativistically-corrected CCSD(T), IC-MRCI and IC-MRCI+ Q have been employed in an investigation of low-lying states of LiH2, NaH2 and KH2. The lowest 2A1 and 2Σ− states were found to be purely repulsive, in agreement with previous predictions. Equilibrium structures of the 2B2, 2B1 and 2Σg+ states have been calculated and are in good agreement with available theoretical data. For the C2v species the main factor determining the structure and stability was the relative orientations and occupations of the valence p atomic orbital of the metal and the H2 1σg orbital. The presence of occupied p orbitals in the metal ligand increased the stability of the D∞h species. Spectroscopic parameters of the 2B2, 2B1 and 2Σg+ states of KH2 are reported here for the first time.