Fluctuation and Functions of Liquid Water

研究了水动力学的各个方面：(1)氢键网络重排的起伏和松弛及其观察；(2)质子在液态水和冰中的迁移；(3)超临界水中的溶剂化动力学；(4)水冻结的机理。我们发现在液体状态下一般不存在五阶非线性响应。(2)(3)我们研究了质子在液态水和冰中的动力学行为。氢键网络重排(HBNR)有助于水中的质子转移，使一些水分子形成三配位。我们还分析了水分子解离的机理。水分子解离有两个通道：离子通道和自由基通道。研究还发现，尽管水的分子密度较小，但超临界水中的水合作用比水中强，因此离子通道变得更加稳定。(4)我们还讨论了水冻结的机理。由于具有包含各种深层能量极小值的“脆弱”特征的势能面[4]，水预计会被困在这些局部极小值中的一个处，并在冷却时形成无定形结构。为了理解水通常是如何冻结成晶体结构的，以及这条通往晶体的途径是如何从那些到无定形冰的分支的，必须分析水的势能面(GPES)的整体性质。作为获得这些知识的第一步，我们检查了水分子团簇的GPES，这与氢键网络如何随着能量的降低而变化有关。然后对水的冻结进行了分子动力学计算。研究发现，在一定的诱导时间后，晶化突然发生。进行了各种分析，以找出氢键网络是如何在这种冻结机制中生长的。

Various aspects of Water Dynamics have been investigated ; (1) Fluctuation and relaxation in hydrogen bond network rearrangement and its observation, (2) Proton transfer in liquid water and ice, (3) Solvation dynamics in supercritical water, and (4) Mechanism of water freezing.(1) was investigated with normal mode analysis and MD calculation. We have found that there is no echo in 5th order nonlinear response in liquid state in general.(2) (3) We have investigated the dynamic behavior of protons in liquid water and ice. Proton transfer in water is assisted by Hydrogen Bond Network Rearrangement (HBNR), making some water molecules three-coordinated. We also analyze the mechanism of the water molecule dissociation. There are two channels in the water molecular dissociation ; ion and radical channels. It is also found that in super-critical water the hydration is stronger than in water in spite of the fact that water molecular density is smaller, and hence the ionic channel becomes even more stable.(4) We will also discuss the mechanism of water freezing. Having a potential energy surface (PES) of the 'fragile' characteristic [4] involving various deep energy minima, water is expected to be trapped at one of these local minima and form an amorphous structure upon cooling. To understand how water usually freezes to a crystalline structure, and how this pathway to a crystal bifurcates from those to amorphous ices, the global nature of the potential energy surface (GPES) of water must be analyzed. As a first step toward obtaining such knowledge, we examine GPES of water molecular clusters, related to how the hydrogen bond network changes with lowering energy. We then performed MD calculation on water freezing. It was found that crystalline suddenly takes place after certain induction-time. Various analyses were made to find how the hydrogen bond network grows in this freezing mechanism.