硫酸和水小分子团簇零点运动的意义
Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water
摘要：
大气中微量气体的粒子成核是云凝结核（CCN）的重要来源，鉴于均匀水滴成核所需的高过饱和度，这些对于云的形成至关重要。量子化学方法由于其在计算构型能方面的高精度和高效率而越来越多地被用于模拟成核;并且成核速率可以从粒子形成的相关自由能获得。然而，即使在这种先进的方法中，通常假设原子核具有经典性质，这对于某些系统来说是有问题的。零点运动（也称为量子核动力学）在模拟硫酸和水的小集群的重要性在这里测试使用路径积分分子动力学（PIMD）方法在密度泛函理论（DFT）的理论水平。零点运动的一般效果是稍微扭曲的平均结构，并促进相对于经典行为的质子转移的程度。在一个硫酸分子与三个水的特定配置中，在300 K下由硫酸和水分子之间的质子探索的位置范围（与0 K下的几何优化所建议的限制相比，范围很宽）明显受到零点运动的影响，并且对于其他配置观察到类似的效果。
英文摘要：
The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei (CCN), and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics (PIMD) method at the density functional theory (DFT) level of theory. The general effect of zero-point motion is to distort the mean structure slightly, and to promote the extent of proton transfer with respect to classical behaviour. In a particular configuration of one sulphuric acid molecule with three waters, the range of positions explored by a proton between a sulphuric acid and a water molecule at 300 K (a broad range in contrast to the confinement suggested by geometry optimisation at 0 K) is clearly affected by the inclusion of zero point motion, and similar effects are observed for other configurations.
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