Proton Transfer in Bulk Water by Full Adaptive QM/MM Method: Integration of Solute- and Solvent-Adaptive Approaches

The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that makes local electronic structures of large systems accessible.
It has the strengths of accuracy found in the QM method as well as the strengths of small
computational costs found in the MM method. However, it is severe to directly apply the
QM/MM method to dynamics of solution systems, particularly to proton transfer. As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ion and solvent water molecules. Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. Although there have been several solvent-adaptive methods proposed, a full adaptive framework, an approach that also takes into account of adaptation for solutes, still remains untapped. In this paper, we propose a new numerical expression for the coordinate of the excess proton and its control algorithm. Furthermore, we confirmed that this method can stably and accurately simulate proton transfer dynamics in bulk water.