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Model first principles molecular dynamics study on the fate of vibrationally excited states in liquid water

Carr–Parrinello MD calculations for a simplistic periodic model of liquid water are performed to probe temperature dependence of infrared activation lifetime. IR activation is classically simulated by adding an appropriate velocity to the proton in a tagged water molecule. The evolution of hydrogen bonding descriptors is monitored through consecutive simulations to spot the onset of qualitative changes in the hydrogen bonding network; they are related to vibrational energy relaxation. The applied ionic simulation temperature (elevated by 20%) decreases the tendency to overbinding characteristic for CP MD calculations. Qualitatively estimated stretch lifetimes are 280, 320 and 400 fs for temperatures of 298, 320 and 370 K, respectively. This work gives direct evidence of the parallel dependence of both the decay of OH activation and the hydrogen bond network on temperature, which offers a viable explanation for the experimentally observable unusual increase in OH excitation lifetime with temperature.