First-principles study of AuS_n (2 ≤ n ≤ 7) clusters: structural, electronic, magnetic, spectral properties, and adsorption properties with O₂ and H₂O

Based on the density functional theory (DFT) method, the geometrical structures, relative stability, electronic, and spectral properties of AuS_n (2 ≤ n ≤ 7) clusters are systematically studied, whose sizes are largely in the range of 0.3–0.5 nm. AuS₄ and AuS₃ show the highest and lowest energy gaps and hence the highest chemical stability and chemical activity, respectively. The total magnetic moment is 3.0 and 1 (or 0) μ_B for AuS₄ and other clusters, respectively, which largely (about 99.9%) arise from the local magnetic moment of S atoms. The average polarization tensor for single atom (<\( \overline{\alpha} \)>) generally increases with increasing the number of S atoms, with the maximum 91.72 a.u. for AuS₇, corresponding to the most significant delocalization effect. AuS₄ cluster exhibits the highest polarizability anisotropic invariant Δα (≈ 350.56 a.u.) and the lowest total dipole moment (≈ 0.06 D) among all systems, corresponding to the strongest anisotropic response to external electric field and the weakest polarization, respectively. The IR, Raman, UV-Vis, and PES spectra are simulated for AuS_n (2 ≤ n ≤ 7) clusters with the structures of the lowest isomers. The O–O and O–H bond lengths, adsorption energies, vibration frequencies, and density of states are also calculated for AuS₄ and AuS₃ adsorbing one O₂ and H₂O molecules, respectively. The adsorption capacity of AuS₃ for both gas molecules is higher than AuS₄, and AuS_n (n = 3, 4) clusters are more favor to adsorb O₂ than H₂O.