Quantum tunnelling in water clusters

计算化学和生物学的许多领域需要精确和计算高效的势能面来描述水分子之间的相互作用。在开发和模拟这种潜力方面已经取得了很大进展，但仍有许多事情需要了解。这一领域的当代重要性从最近的实验产生的新活动中可见一斑，而新的瞬子理论为量子动力学计算提供的机会表明，现在将有可能取得快速进展。水团簇研究原则上是开发和提炼水势的一项非常强大的技术。尽管这种团簇的动力学可能与整体水的动力学相去甚远，但水分子之间的相互作用当然是相同的，研究水团簇的优势是它们是在非常低的温度下在分子束中制备的，因此可以进行精确和详细的光谱测量，这对水势的性质做出了敏感的反应。如果一个人能发展出一种方法来根据电势计算这些光谱线，那么人们就在水势和实验之间建立了强有力的、直接的联系。开发这样一种方法，并将其应用于包含4到至少20个水分子的团簇，是这项拟议研究的主要目标。我们将研究的特殊跃迁是那些涉及水团簇的不同置换异构体之间的量子隧道的跃迁。这种分析将允许我们使用一种新的‘瞬子’方法，这是一种系统地获得对主要隧道路径的良好近似的方法。该方法已经在水二聚体和三聚体上进行了测试，并获得了良好的重复性实验结果。这项拟议的研究将进一步加强和发展这些技术，使它们能够应用于包含多达20个水分子的星系团。这项工作将导致对这些星系团的隧道分裂模式的首次预测，然后将其与项目合作伙伴Rich Saykally(美国伯克利)小组的实验测量结果进行比较。这些比较将使我们能够改进和提炼水的势能面，这将与项目合作伙伴Joel Bowman(美国埃默里)合作进行。除了水簇，我们还将研究水与碳氢化合物的络合物。这项工作将产生更好的势能面来描述天然气水合物中的相互作用，这将导致对这些系统更可靠的模拟，以及与全球变暖和替代能源储量开发相关的新结果。一家石油咨询软件公司(InfoChem)对这项工作可能产生的发展非常感兴趣，并被指定为我们的项目合作伙伴之一。Saykally小组已经获得了水-甲烷等碳氢化合物络合物的高分辨率光谱，我们的计算将随着实验的反馈而继续进行。这项工作对水势的改进可能会导致对水在其所有相的更可靠的模拟，从而导致更好地表示和理解包含水的广泛的重要化学和生物体系。

Many areas of computational chemistry and biology require accurate and computationally efficient potential energy surfaces to describe the interactions between water molecules. A great deal of progress has been made in developing and modelling such potentials, but much remains to be understood. The contemporary importance of this field is evident from new activity generated by recent experiments, and the opportunity afforded by novel instanton theory for quantum dynamics calculations suggests that rapid progress will now be possible. The study of water clusters is in principle a very powerful technique for developing and refining water potentials. Although the dynamics of such clusters may be far from that of water in the bulk, the interactions between the water molecules are of course the same, and the advantage of studying water clusters is that they are prepared at very low temperatures in a molecular beam, thus allowing precise and detailed spectroscopic measurements to be made, which respond sensitively to the properties of the water potential. If one can develop a method for computing these spectral lines from the potentials, then one has established a powerful, direct, link between the water potential and experiment. Developing such a method, and applying it to clusters containing from 4 to at least 20 water molecules is the primary goal of this proposed research. The particular transitions that we will study are those that involve quantum tunnelling between different permutational isomers of the water clusters. This analysis will allow us to use a novel 'instanton' method, which is a systematic way of obtaining a good approximation to the dominant tunnelling paths. This method has already been tested on water dimer and trimer, and shown to give excellent results that reproduce experiment. The proposed research will augment and develop further these techniques, permitting them to be applied to clusters containing up to around 20 water molecules. This work will result in the first predictions of the tunnelling splitting patterns for these clusters, which will then be compared with experimental measurements made in the group of project partner Rich Saykally (Berkeley, USA). These comparisons will then allow us to improve and refine the water potential energy surface, which will be conducted in collaboration with project partner Joel Bowman (Emory, USA).In addition to water clusters, we will also study complexes of water with hydrocarbons. This work will result in better potential energy surfaces for describing the interactions in gas hydrates, which will lead to more reliable simulations of these systems and new results that will be relevant to studies of global warming and exploitation of alternative energy reserves. An oil consultancy software company (InfoChem) is very interested in possible developments resulting from this work, and is named as one of our project partners. High resolution spectra for hydrocarbon complexes such as water-methane have already been obtained in the Saykally group, and our calculations will be carried forward with ongoing feedback from experiment.The improvements to the water potentials that result from this work are likely to lead to more reliable simulations of water in all its phases, and thus to lead to better representations and understanding of the vast range of important chemical and biological systems that contain water.