Molecular rotation-caused autocorrelation behaviors of thermal noise in water

The finite autocorrelation time of thermal noise is crucial to unidirectional transportation on the molecular scale. Therefore, it is important to understand the cause of the intrinsic picosecond autocorrelation time of thermal noise in water. In this work, we use molecular dynamics simulations to compare the autocorrelation behaviors of the thermal noise, hydrogen bonds, and molecular rotations found in water. We found that the intrinsic picosecond autocorrelation time for thermal noise is caused by finite molecular rotation relaxation, in which hydrogen bonds play the role of a bridge. Furthermore, the simulation results show that our method of calculating the autocorrelation of thermal noise, by observing the fluctuating force on an oxygen atom of water, provides additional information about molecular rotations. Our findings may advance the understanding of the anomalous dynamic nanoscale behavior of particles, and the applications of terahertz technology in measuring the structural and dynamical information of molecules in solutions. (authors)