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 (1 1 1)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 (1 1 1)H are 1.512(1.524) Å, 2668(2647) cm−1, and 2.87(2.77) eV for Re, ωe, and De, respectively. The calculated spin–orbit effects (+0.009 Å,−113 cm−1, and −0.64 eV) are modest, although the molecule has a closed-shell electronic structure.