Comparison of classical and ab initio simulations of hydronium and aqueous proton transfer.

Abstract

Proton transfer represents the simplest chemical reaction, yet it is still typically not accounted for in standard biomolecular simulations. Here we compare an ab initio molecular dynamics (MD) simulation trajectory of an excess proton in water to classical MD trajectories of hydronium in TIP3P water. We characterize the structural differences between them and correlate these with the energy gap upon instant proton transfer, which is much higher in the classical simulations. One structural difference is in the tilt angle of the waters that accept an h-bond from hydronium, which is too large in TIP3P. This can be largely alleviated by use of the TIP5P model, whose lone pairs orient the water molecule properly. Other discrepancies include a lower number of h-bonds and a less favorable second solvation shell geometry in classical MD, which is likely due to lack of polarization. The identification of these structural differences paves the way toward classical MD simulations that can more closely mimic ab initio trajectories. We also present efforts to develop a classical model for hydroxide that reproduces ab initio solvation structure and proton transfer mechanism.