POTENTIAL-ENERGY SURFACES FOR INSERTION OF HF ATOMS INTO H-2

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by second-order CI (SOCI) and relativistic CI (RCI) calculations including spin-orbit coupling are carried out on low-lying electronic states of HfH2. Potential energy surfaces of 12 electronic states of HfH2 for studying the insertion of Hf(a3F), Hf(a1D), and Hf(a5F) states into H2 are constructed. We find that the low-lying Hf(1D) inserts into H2 spontaneously to form the HfH2 molecule in the 1A1 state, while the 3F ground state of Hf atom has to surmount a barrier for insertion into H2. However, the small but nonnegligible spin-orbit coupling matrix elements at the crossing of the 1A1 bent-structure surface with triplet surfaces provides a nonzero nonadiabatic transition probability for insertion of the Hf(a3F) state with a smaller barrier into H2. The 1A1 ground state of HfH2 was found to have r(e) = 1.842 angstrom, theta-e = 126.7-degrees, mu-e = 0.31 D, and D(e) = 30 kcal/mol relative to Hf(3F2) + H2. There are several low-lying triplet and singlet states that mix with each other in the presence of spin-orbit coupling.