Ionisation and (de-)protonation energies of gas-phase amino acids from an optimally tuned range-separated hybrid functional

We provide a quantitative examination of the ionisation potential, protonation, and de-protonation energies of a set of 22 different amino acids. Specifically, we compare results obtained using the conventional hybrid functionals B3LYP and BHLYP (which use 20% and 50% of Fock exchange, respectively) with those obtained from the recently developed optimally tuned range-separated hybrid (OT-RSH) functional approach, as well as to literature coupled-cluster calculations with single and double excitations (CCSD) data. We find the OT-RSH results to be quantitatively close to those of the BHLYP functional and a significant improvement over those of B3LYP, with respect to the CCSD data. We conclude that the results of the OT-RSH for these quantities are as reliable and inexpensive as those of BHLYP, but possess one distinct and important advantage: they overcome the empiricism and limited predictive power associated with the arbitrary choice of the amount of Fock exchange in the BHLYP. We further discuss quantitative and qualitative trends in the optimal-tuning procedure of the amino acids studied.