ECONOMICAL TREATMENTS OF RELATIVISTIC EFFECTS AND ELECTRON CORRELATION IN WH6

The equilibrium geometries and relative stabilities of several structu ral isomers of tungsten hexahydride, WH6, have been obtained at differ ent levels of quantum chemical calculations. The performance of variou s strategies to (i) include electron correlation, viz. density functio nal theory based approaches, Moller/Plesset perturbation and coupled c luster theory, and to (ii) account for scalar relativistic effects, vi z. various relativistic effective core potentials, first order perturb ation theory, a quasi-relativistic treatment employing a Pauli Hamilto nian, and use of the Douglas/Kroll operator, are compared to the best theoretical data available. It is shown that relativistic and electron correlation effects are most important for the high-symmetry species, that these effects give rise to opposite trends in relative energies, and that overall the relativistic effects dominate. The most efficien t way to incorporate relativistic effects appears to be via the use of relativistic effective core potentials, while the correlation energie s are best taken account of using a conventional method such as CCSD(T ). (C) 1998 John Wiley & Sons, Inc.