Geometries, Binding Energies, Ionization Potentials, and Electron Affinities of Metal Clusters: Mg _n 0, ± ¹, n = 1-7

Equilibrium geometries, binding energies, adiabatic ionization potentials, and adiabatic electron affinities for neutral and singly charged magnesium clusters, Mg_n0, ± ¹, n = 1-7, have been computed using 39 exchange-correlation (XC) functionals in Kohn-Sham density functional theory and several coupled-cluster methods with single, double, and triple excitations, including CCSD(T) for all species, CCSD(2)_T and CR-CC(2,3) for species with n = 1-3, and CCSDt, CC(t;3), and CCSDT for species with n = 1 and 2. We have used augmented polarized-valence and polarized-core-valence correlation-consistent basis sets. We have found that the geometry and binding energy of the weakly bound Mg₂ dimer requires a robust treatment of connected triple excitations, represented in this work by the CR-CC(2,3), CC(t;3), and full CCSDT methods, which are more accurate than the popular quasi-perturbative CCSD(T) approximation, but CCSD(T) is sufficiently accurate to be applied to other Mg clusters. We have also demonstrated that for all Mg clusters examined in this study, hybrid XC functionals generally have higher accuracy than local ones, with PW6B95, SOGGA11-X, M11, and PWB6K being the most accurate, both for the geometries and for the binding energies, ionization potentials, and electron-detachment energies.