Two-component coupled-cluster calculations for the hydride of element 111: On the performance of relativistic effective core potentials

We have calculated bond lengths, harmonic vibrational frequencies, and dissociation energies for (111)H using relativistic effective core potentials (RECP) including one-electron spin-orbit operators at the Hartree-Fock and coupled-cluster levels of theory. The spectroscopic constants calculated using shape-consistent RECPs compare favorably with available four-component results, but energy-consistent RECPs are found to underestimate the spin-orbit effects. The best computed(estimated) spectroscopic constants of (111)H are 1.512(1.524)Å,2668(2647)cm^-1, and 2.87(2.77) eV for R_e, ω_e, and D_e, respectively. The calculated spin-orbit effects (+0.009Å,-113cm^-1, and -0.64 eV) are modest, although the molecule has a closed-shell electronic structure.