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We studied acetylhistidine (AcH), bare or microsolvated with a zinc cation bysimulations in isolation. First, a global search for minima of the potentialenergy surface combining both, empirical and first-principles methods, isperformed individually for either one of five possible protonation states.Comparing the most stable structures between tautomeric forms of negativelycharged AcH shows a clear preference for conformers with the neutral imidazolering protonated at the N-epsilon-2 atom. When adding a zinc cation to thesystem, the situation is reversed and N-delta-1-protonated structures areenergetically more favorable. Obtained minima structures then served as basisfor a benchmark study to examine the goodness of commonly applied levels oftheory, i.e. force fields, semi-empirical methods, density-functionalapproximations (DFA), and wavefunction-based methods with respect to high-levelcoupled-cluster calculations, i.e. the DLPNO-CCSD(T) method. All tested forcefields and semi-empirical methods show a poor performance in reproducing theenergy hierarchies of conformers, in particular of systems involving the zinccation. Meta-GGA, hybrid, double hybrid DFAs, and the MP2 method are able todescribe the energetics of the reference method within chemical accuracy, i.e.with a mean absolute error of less than 1kcal/mol. Best performance is foundfor the double hybrid DFA B3LYP+XYG3 with a mean absolute error of 0.7 kcal/moland a maximum error of 1.8 kcal/mol. While MP2 performs similarly asB3LYP+XYG3, computational costs, i.e. timings, are increased by a factor of 4in comparison due to the large basis sets required for accurate results.

Relative energetics of acetyl-histidine protomers with and without Zn2+ and a benchmark of energy methods