Calculation of bond energies in compounds of heavy elements by a quasi-relativistic approach

Abstract

A quasi-relativistic method, in which the valence density is optimized with respect to the first-order relativistic Hamiltonian, has been evaluated by calculations on systems containing heavy elements including third-row transition metals and actinides. The method adopts the statistical energy expression and employs in addition the frozen core approximation. The quasi-relativistic method has been applied in calculations on atomic orbital energies for the valence shells of heavy elements. It is concluded from these calculations that the quasi-relativistic scheme affords results in better accord with the fully relativistic Dirac-Slater method than the first-order relativistic method based on perturbation theory. Calculations on the M-X bond energies in MX{sub 4} (M = Th, U; X = F, Cl, Br, I) as well as the M-R bond energies in Cl{sub 3}MR (M = Th, U; R = H, CH{sub 3}) revealed in addition that bond energies based on the quasi-relativistic method (QR) were in better agreement with experimental data than bond energies based on the first-order perturbation theory (FO). The absolute mean derivations with respect to experimental values were 6.9 and 16.5 kcal mol{sup {minus}1} for QR and FO, respectively, the case of the MX{sub 4} systems. It is concluded that the quasi-relativistic method, inmore » which changes in the electron density induced by relativity ({Delta}{rho}{sup R}) are approximately taken into account in the energy expression, should be used for compounds containing actinides. Both QR and FO (in which contributions from {Delta}{rho}{sup R} to the total energy are absent, even though they are present in the orbital energies) are appropriate for elements up to Z = 80, although QR represents a slight improvement for the elements in the third transition series.« less