Large local energy fluctuations in water

A detailed analysis is made for the dynamical behavior of an individual water molecule in liquid water by using a classical molecular dynamics (MD) calculation. It is found that there exist very large potential energy fluctuations in water; a single water molecule can exhibit a fluctuation of the order of 10∼20 kcal/mol. These potential energy fluctuations can be classified into two categories; the fast component (10^-14-10 ^-13 s) associated with librational motions of water molecules and the slow component (10^-12-10^-11 s) associated with water binding structure changes. Both amplitudes can be reached up to 10 kcal/mol. Due to strong Coulomb (dipole-dipole) interaction, small mutual geometrical changes, caused by the libration motions, induce large fast potential energy fluctuations. Due to large cohesion energy of the hydrogen bond and the nature of the water binding structure, there exist many water pair interactions which are unattactive or even repulsive; the water molecule potential (binding) energy distribution is inhomogeneous in space. This inhomogeneity alters as the water structure changes. Each water molecule then makes a transition from a stable binding to an unstable one with surrounding molecules and vice versa, inducing large potential energy changes in the picosecond order. A detailed analysis is made concerning how the hydrogen bonding pattern changes with water molecular motions. Significance of the existence of large water potential energy fluctuations on chemical processes is discussed.