Ab initio fully relativistic molecular calculations: bonding in gold hydride

The electronic structure of gold hydride is investigated by ab initio fully relativistic extended basis set self-consistent field and configuration interaction calculations based on the Dirac equation. The gold 6p orbitals play only a very minor role in the bonding. The ten electrons occupying the 5d orbitals in the free gold atom are significantly affected by the formation of the molecule whose electronic structure exhibits substantial 5d-6s hybridization. The extended-basis calculations show that relativity shortens the bond length by 0.45 a.u. (1 a.u. (atomic unit) of length = 1 bohr $\approx$ 0.529177 x 10$^{-10}$ m), substantially increases the fundamental vibration frequency and doubles the binding energy predicted by using a single determinant wavefunction. The bonding cannot be fully understood by using nonrelativistic theory.