Binding of aluminium to coinage metals: electron correlation and relativistic effects

Abstract

Relativistic coupled cluster calculations have been carried out for diatomic molecules formed by aluminium and the coinage metal atoms (Me). The relativistic effects are accounted for by using the spin-free Douglas–Kroll formalism while the electron correlation contribution is calculated at the level of the CCSD(T) method. The theoretical data are in satisfactory agreement with the known experimental values and available results of other calculations. The AlAu molecule with the calculated relativistic dissociation energy of 3·41 eV is the most stable species in the series. The spectroscopic parameters determined are analysed in terms of the electron correlation and relativistic contributions to their values. These results parallel those obtained recently for a similar series of boron compounds. Dipole and quadrupole moments also were investigated as was the parallel component of the dipole polarizability of all three AlMe molecules, with particular attention given to their decomposition into electron correlation and relativistic contributions. Within the one electron non-relativistic approximation all these molecules turn out to have negative dipole moments (polarity Al(-) Me(-)) whose sign is changed already by taking into account the electron correlation. The relativistic effects enhance the positive dipole moment values and make AlAu the most polar molecule in the series, with a dipole moment of about 1·58 D. Similarly, due to both electron correlation and relativistic contributions, the quadrupole moment of AlAu is positive and differs by more than 7 au from the value calculated in the one-electron non-relativistic approach. The present results obtained without the spin-free Douglas–Kroll formalism are compared with those calculated by using the spin-free (MVD) Pauli correction for relativistic effects. It follows that for systems as heavy as AlAu the MVD correction to non-relativistic results becomes insufficient. This is partly because of the violation of the low-order perturbation treatment of relativistic effects and partly due to the increasing importance of non-additive relativistic-correlation contributions.