Four
established ReaxFF force fields, trained on biochemical systems, have
been systematically benchmarked on 20 proteinogenic amino acids and
11 dipeptides. The force fields were compared with respect to geometries,
energetics, and atomic charges of conformers for the amino acids.
To assess the performance with respect to reactivity, the condensation
reactions for the formation of dipeptides were investigated by calculating
the reaction energetics and pathways. We found systematic errors in
the torsion angles for the amino acids, with deviations over 100°,
and a generally incorrect account of relative energies for amino acid
conformers. In describing the reactivity, only one of the force fields
could reproduce the reaction energies of amino acid condensations
quantitatively. All four force fields predict unphysical mechanisms
for these reactions, involving highly unstable intermediate structures,
proton transfers involving aliphatic protons, and even five-coordinate
carbon atoms. The corresponding energy landscapes exhibit fluctuations
on small length scales and artificial minima.