We present a comparative assessment of the performance of the M06 suite of density functionals (M06, M06-2X, and M06-HF) against an MP2 benchmark for calculating the relative energies and geometric structures of the Cl(-)·arginine and Br(-)·arginine halide ion-amino acid clusters. Additional results are presented for the popular B3LYP density functional. The Cl(-)·arginine and Br(-)·arginine complexes are important prototypes for the phenomenon of anion-induced zwitterion formation. Results are presented for the canonical (noncharge separated) and zwitterionic (charge separated) tautomers of the clusters, as well as the numerous conformational isomers of the clusters. We find that all of the M06 functions perform well in terms of predicting the general trends in the conformer relative energies and identifying the global minimum conformer. This is in contrast to the B3LYP functional, which performed significantly less well for the canonical tautomers of the clusters where dispersion interactions contribute more significantly to the conformer energetics. We find that the M06 functional gave the lowest mean unsigned error for the relative energies of the canonical conformers (2.10 and 2.36 kJ/mol for Br(-)·arginine and Cl(-)·arginine), while M06-2X gave the lowest mean unsigned error for the zwitterionic conformers (0.85 and 1.23 kJ/mol for Br(-)·arginine and Cl(-)·arginine), thus providing insight into the types of physical systems where each of these functionals should perform best.

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Ultraviolet charge-transfer-to-solvent (CTTS) bands are general spectral features of inorganic salts dissolved in dipolar (e.g., aqueous) solvents. The bands originate from excitations of the anions, and the excited states are thought to involve a delocalized electronic state supported by the arrangement of solvent molecules around the ion in the ground state. These diffuse states have fascinated physical chemists since the 1930s, who first used continuum models to explain the phenomenon and then later refined the treatment to include local molecular interactions. In this paper, we report that CTTS bands evolve from features present in very small gas phase clusters, I−⋅(H2O)n, n=1–4, and discuss how localization of the continuum wave function of the ejected electron acts as a precursor to the bound excited state in the liquid.

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