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 diss ociation energies for (111)H using relativistic effective core potentials ( RECP) including one-electron spin-orbit operators at the Hartree-Fock and c oupled-cluster levels of theory. The spectroscopic constants calculated usi ng shape-consistent RECPs compare favorably with available four-component r esults, but energy-consistent RECPs are found to underestimate the spin-orb it 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, om ega(e), and D-e, respectively. The calculated spin-orbit effects (+0.009 An gstrom, -113 cm(-1), and -0.64 eV) are modest, although the molecule has a closed-shell electronic structure. (C) 2000 Elsevier Science B.V. All right s reserved.