Large-scale electron correlation calculations in the framework of the spin-free dirac formalism: the Au2 molecule revisited

Based upon the exact separation of the Dirac-Coulomb Hamiltonian into a spin-free and a spin-dependent part, we present high-level correlated calculations using the spin-free formulation. A newly-implemented large-scale configuration interaction program and the coupled-cluster method are applied to the gold dimer, the ground state of which is determined by electron correlation and scalar relativistic effects. We compare the results to results obtained with the approximate scalar relativistic Douglas-Kroll-Hess Hamiltonian obtained by truncated expansions in unitary transformations of the Dirac-Coulomb Hamiltonian. Furthermore, we determine the effect of spin-orbit coupling on the ground state properties in a coupled-cluster approach and briefly discuss the problem of separating scalar relativistic and spin-orbit effects in the present context.