Simulating the Hydration Structure of Low- and High-Spin [Fe(bpy)3]2+: Long-Range Dispersion and Many-Body Effects

When characterizing transition metal complexes and their functionalities, the importance of including the solvent as an active participant in both theoretical and experimental models is being realized more and more often. Whereas many studies have evaluated long-range dispersion effects inside organic molecules and organometallics, less is known about their role in solvation. Here, we have analysed the components within solute-solvent interactions of one of the most central iron-based photoswitch model systems, in two spin states. We find that long-range dispersion effects modulate the coordination significantly, and that this is accurately captured by density functional theory models including dispersion corrections. We furthermore correlate gas-phase relaxed complex-water clusters to thermally averaged molecular densities. This shows how the gas-phase interactions translate to solution structure as sampled by radial distribution functions, and we show that finite-size simulation cells can cause the radial distribution functions to have artificially enlarged amplitudes. Finally, we quantify the effects of many-body interactions within the solvent shells, and found that almost a fifth of the total interaction energy of the solute-shell system in the high-spin state comes from many-body contributions, which cannot be described by pair-wise additive force field methods.