We present an extension to the ring polymer instanton (RPI) method that includes overall rotations in the tunneling pathways, allowing a calculation of the full rotation-tunneling spectrum of the water dimer. Our method gives a qualitative description of the rotational and tunneling processes underpinning the spectrum, and shows the drastic reduction of the largest splitting with increasing rotational excitation. We show that this reduction is due to the strong coupling between the rotational motion about the principal axis and the acceptor tunneling motion (where the acceptor monomer rotates about the hydrogen bond), which results in a large increase in the path length and hence the quantum action. Our method gives a clear physical understanding of the behaviour of tunneling splittings in the presence of rotations, and scales linearly with system size.

中文翻译：

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从瞬时理论出发，水二聚体的旋转隧道光谱†

我们提出了一种对环聚合物瞬子（RPI）方法的扩展，该方法包括在隧道通道中进行整体旋转，从而可以计算出水二聚体的整个旋转隧道光谱。我们的方法对支撑光谱的旋转和隧穿过程进行了定性描述，并显示了随着旋转激励的增加，最大裂隙的急剧减少。我们表明，这种减少是由于绕主轴线的旋转运动与受体隧穿运动（其中受体单体绕氢键旋转）之间的强耦合引起的，从而导致光程长度增加，从而导致量子行动。我们的方法对存在旋转的隧道裂隙的行为提供了清晰的物理理解，