Fluctuation and Functions of Liquid Water

研究了水动力学的各个方面：（1）氢键网络重排的波动和弛豫及其观察，（2）液态水和冰中的质子转移，（3）超临界水中的溶剂化动力学，（4）水冻结的机理。(1)通过简正模态分析和分子动力学计算，研究了该结构的振动特性。我们发现，在液体状态下，五阶非线性响应一般不存在回波。(2)（3）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）有助于质子在水中的转移，使一些水分子成为三配位的。并对水分子的解离机理进行了分析。水分子的解离有两个通道：离子通道和自由基通道。研究还发现，在超临界水中，尽管水分子密度较小，但水合作用比在水中强，因此离子通道变得更加稳定。(4)我们还将讨论水冻结的机制。由于势能面（PES）具有“脆弱”特征[4]，涉及各种深层能量极小值，预计水将被捕获在这些局部极小值之一，并在冷却时形成无定形结构。为了理解水通常如何冻结成晶体结构，以及这种晶体的路径如何从非晶冰分叉，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，我们研究了水分子簇的GPES，与氢键网络如何随能量降低而变化有关。然后，我们进行了水冻结的MD计算。发现在一定的诱导时间后，晶化现象突然发生。进行了各种分析，以发现氢键网络如何在这种冻结机制中生长。

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.