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) Angstrom, 2668(2647) cm(-1), and 2.87(2.77) eV for R-e, omega(e), and D-e, respectively. The calculated spin-orbit effects (+0.009 Angstrom, -113 cm(-1), and -0.64 eV) are modest, although the molecule has a closed-shell electronic structure.