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效率高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，但仍有许多问题有待了解。这个领域的当代重要性是显而易见的，从最近的实验产生的新活动，和新的瞬子理论提供的量子动力学计算的机会表明，快速的进展现在将是可能的。水分子团簇的研究在原则上是一个非常强大的技术开发和完善水的潜力。虽然这种团簇的动力学可能与水分子的动力学相差甚远，但水分子之间的相互作用当然是相同的，研究水分子团簇的优势在于它们是在分子束中在非常低的温度下制备的，因此可以进行精确和详细的光谱测量，这对水势的性质敏感。如果人们能发展出一种方法，从水势计算这些谱线，那么人们就在水势和实验之间建立了一种强有力的、直接的联系。开发这样一种方法，并将其应用于包含4至至少20个水分子的簇是这项研究的主要目标。我们将研究的特定的过渡是那些涉及水簇的不同置换异构体之间的量子隧穿。这种分析将使我们能够使用一种新的“瞬子”方法，这是一种系统的方式获得一个很好的近似占主导地位的隧道路径。该方法已经在水二聚体和三聚体上进行了测试，并显示出重现实验的优异结果。拟议中的研究将进一步增强和发展这些技术，使它们能够应用于包含多达20个水分子的团簇。这项工作将导致这些集群的隧道分裂模式的第一个预测，然后将与项目合作伙伴Rich Saykally（美国伯克利）的实验测量进行比较。这些比较将使我们能够改进和完善水势能面，这将与项目合作伙伴Joel Bowman（Emory，USA）合作进行。除了水簇，我们还将研究水与碳氢化合物的复合物。这项工作将产生更好的势能面，用于描述天然气水合物中的相互作用，这将导致对这些系统进行更可靠的模拟，并产生与全球变暖研究和开发替代能源储备相关的新结果。一家石油咨询软件公司（InfoChem）对这项工作可能带来的发展非常感兴趣，并被指定为我们的项目合作伙伴之一。赛卡利小组已经获得了水-甲烷等碳氢化合物复合物的高分辨率光谱，我们的计算将根据实验的持续反馈进行下去。这项工作对水势的改进可能会导致对水的所有相态进行更可靠的模拟，从而导致更好地表示和理解包含水的大量重要化学和生物系统。

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.