Using Constrained Density Functional Theory to Track Proton Transfers and to Sample Their Associated Free Energy Surface

Ab initio molecular
dynamics (AIMD) and quantum mechanics/molecular
mechanics (QM/MM) methods are powerful tools for studying proton solvation,
transfer, and transport processes in various environments. However,
due to the high computational cost of such methods, achieving sufficient
sampling of rare events involving excess proton motionespecially
when Grotthuss proton shuttling is involvedusually requires
enhanced free energy sampling methods to obtain informative results.
Moreover, an appropriate collective variable (CV) that describes the
effective position of the net positive charge defect associated with
an excess proton is essential both for tracking the trajectory of
the defect and for the free energy sampling of the processes associated
with the resulting proton transfer and transport. In this work, such
a CV is derived from first principles using constrained density functional
theory (CDFT). This CV is applicable to a broad array of proton transport
and transfer processes as studied via AIMD and QM/MM simulations.