Application of a kinetic energy partitioning scheme for ab initio molecular dynamics to reactions associated with ionization in water tetramers

We analyze the short-time dynamics of 'cyclic' and 'branched' water tetramers after an ionization event, with the aid of a scheme that partitions the kinetic energy of a solute plus solvent system into separate solute and solvent (or bath) contributions, using instantaneous internal coordinates and atomic velocities. The analysis supports the partitioning of the tetrameric systems into two subsystems, a 'reactive trimer' and a 'solvent' molecule. The partitioned kinetic energy exhibits two features, a broad peak assigned to the interaction between the two sub-systems and a sharper peak arising from the proton transfer that occurs upon ionization. It is found that the stability of the hydroxyl radical formed upon ionization is sensitive to the configuration of the water molecules around the ionized water at the moment of the ionization event.