Multireference study of spin-orbit coupling in the hydrides of the 6p-block elements using the model core potential method.

Careful spin-orbit multireference studies were carried out for the late p-block elements Tl, Pb, Bi, Po, At, and Rn and their hydrides using the model core potentials developed in the present work. The model core potentials were designed to treat the scalar-relativistic and spin-orbit coupling effects at the Douglas-Kroll level. The variational stability of the spin-orbit coupling operator was discussed in terms of the relativistic kinematic operators and depicted graphically. A detailed analysis of the spin-orbit multireference dissociation curves of the 6p element hydrides as well as of their atomic spectra allowed to establish the accuracy of the model core potentials with respect to all-electron calculations to be within several mA for r(e), meV (ceV) for D(e) at the correlation level of configuration interaction (multireference perturbation theory), 30 cm(-1) for omega(e), and about 350 cm(-1) for the low-lying atomic and molecular term and level energies. These values are expected to be the maximum error limits for the model core potentials of all the np-block elements (n=2-6). Furthermore, a good agreement with experiment requires that many terms be coupled in the spin-orbit coupling calculations. A timing study of Tl and TlH computations indicates that the model core potentials lead to 20-fold (6-fold) speedup at the level of configuration interaction (multireference perturbation theory) calculations.