Ab initio relativistic pseudopotential study of small silver and gold sulfide clusters (M2S)n, n=1 and 2

Small semiconductor silver and gold sulfide clusters (Ag2S)n and (Au2S)n, n=1,2, are studied by valence ab initio calculations with the inclusion of electron correlation at the second-order perturbation theory (MP2) and coupled-cluster [CCSD and CCSD(T)] levels. Various relativistic and nonrelativistic pseudopotentials are employed to describe the effects of core electrons. Correlation and relativistic effects are essential in determining the geometry and relative stability of monomer and dimer structures. Relativistic effects result in a notable decrease in the calculated interatomic distances, which is especially significant in the case of gold sulfide structures (up to 10%). Correlation effects markedly increase the stability of compact structures with an increased number of relatively short M…M contacts (M…M distances of about 280–330 pm). Excluding the correlation of lower-lying valence orbitals (sulfur 3s and silver 4d or gold 5d) results in completely opposite predictions. This fact suggests that the effects of d–d and d–outer valence (metal ns and sulfur 3p) electron correlation give rise to attractive short-range interactions of intramolecular van der Waals type, which determine the increased stability of more compact cluster structures. However, large-core pseudopotentials strongly exaggerate this effect in the case of gold and give results rather different from those obtained with more valid and accurate small-core pseudopotentials. It is shown that the reason for this deficiency lies in the nature of pseudopotentials themselves rather than in basis-set shortcomings. The atomization and dissociation energies, equilibrium geometrical parameters, dipole moments, and Mulliken populations are calculated and discussed.