G₀W₀ Ionization Potentials of First-Row Transition Metal Aqua Ions

We report computations of the vertical ionization potentials within the GW approximation of the near-complete series of first-row transition metal (V-Cu) aqua ions in their most common oxidation states, i.e., V³⁺, Cr³⁺, Cr²⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺. The d-orbital occupancy of these systems spans a broad range from d² to d⁹. All of the structures were first optimized at the density functional theory level using a large cluster of explicit water molecules that are embedded in a continuum solvation model. Vertical ionization potentials were computed with the one-shot G₀W₀ approach on a range of transition metal ion clusters (6, 18, 40, and 60 explicit water molecules), wherein the convergence with respect to the basis set size was evaluated using the systems with 40 water molecules. We assess the results using three different density functional approximations as starting points for the vertical ionization potential calculations, namely, G₀W₀@PBE, G₀W₀@PBE0, and G₀W₀@r²SCAN. While the predicted ground-state structures are similar to all three exchange-correlation functionals, the vertical ionization potentials were in closer agreement with experiment when using the G₀W₀@PBE0 and G₀W₀@r²SCAN approaches, with the r²SCAN-based calculations being significantly less expensive. Computed bond distances and vertical ionization potentials for all structures are in good agreement with available experimental data.