Ab initio molecular dynamics simulation of the energy-relaxation process of the protonated water dimer

Relaxation processes of the energy-rich protonated water dimer H ⁺(H₂O)₂ were investigated by the ab initio molecular dynamics (AIMD) method. At first, the energy-rich H⁺(H ₂O)₂ was reproduced by simulating a collision reaction between the protonated water monomer H₃O⁺ and H ₂O. Next it was collided with N₂ in order to observe the effects of intramolecular vibration redistribution and intermolecular energy transfer. Forty-eight AIMD simulations of the collision of H^-(H ₂O)₂ with N₂ were performed by changing the initial orientation and the time interval between two collisions. It was revealed that the amount of energy transferred from H⁺(H ₂O)₂ to N₂ decreased the longer the time interval. The relationship between the intermolecular energy transfer and the vibrational states was examined with the use of an energy-transfer spectrogram (ETS), which is an analysis technique combining energy density analysis and short-time Fourier transform. The ETS demonstrates a characteristic vibrational mode for the energy transfer, which corresponds to the stretching of the hydrogen bond between H⁺(H₂O)₂ and N ₂ in an active complex.