Competing quantum effects in the dynamics of a flexible water model
Competing quantum effects in the dynamics of a flexible water model
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DOI：
10.1063/1.3167790
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发表时间：
2009-07-14
影响因子：
4.4
通讯作者：
Manolopoulos, David E.
中科院分区：
文献类型：
--
作者：
Habershon, Scott;Markland, Thomas E.;Manolopoulos, David E.
许多研究已经确定了液态水动力学中的大量子力学效应。在本文中，我们认为，这些影响可能被高估，由于使用刚性水模型和灵活的模型，其中分子内的相互作用，使用简单的谐波函数描述。为了证明这一点，我们引入了一个新的简单的点电荷模型的液态水，q-TIP 4P/F，其中的O-H拉伸所描述的莫尔斯型函数。我们已经参数化了这个模型，以给出正确的液体结构，扩散系数和红外吸收频率的量子（基于路径积分）模拟。该模型还再现了液体密度的实验温度变化，并提供了合理的协议，在大气压下的六角形冰的实验熔化温度。通过比较经典和量子模拟的液体，我们发现，量子力学的波动增加了我们的模型中的平移扩散和取向弛豫率的一个因素约1.15。这种效应远小于在所有先前的经验水模型的模拟中观察到的效应，这些经验水模型已经发现了至少1.4的量子效应，而不管所采用的量子模拟方法或水模型。我们模型中的小量子效应是两种竞争现象的结果。分子间零点能和隧道效应使氢键网络不稳定，导致具有较大扩散系数的较低粘性液体。然而，这被分子内零点运动所抵消，这改变了平均水单体的几何形状，导致更大的偶极矩，更强的分子间相互作用和更慢的扩散。最后，我们建议，其他势能模型的模拟的基础上，我们发现在扩散系数的小量子效应与我们的模型产生一个单一的广泛的O-H伸缩带的红外吸收光谱的能力。(C)2009年美国物理学会。[DOI 10.1063/1.3167790]
Numerous studies have identified large quantum mechanical effects in the dynamics of liquid water. In this paper, we suggest that these effects may have been overestimated due to the use of rigid water models and flexible models in which the intramolecular interactions were described using simple harmonic functions. To demonstrate this, we introduce a new simple point charge model for liquid water, q-TIP4P/F, in which the O-H stretches are described by Morse-type functions. We have parametrized this model to give the correct liquid structure, diffusion coefficient, and infrared absorption frequencies in quantum (path integral-based) simulations. The model also reproduces the experimental temperature variation of the liquid density and affords reasonable agreement with the experimental melting temperature of hexagonal ice at atmospheric pressure. By comparing classical and quantum simulations of the liquid, we find that quantum mechanical fluctuations increase the rates of translational diffusion and orientational relaxation in our model by a factor of around 1.15. This effect is much smaller than that observed in all previous simulations of empirical water models, which have found a quantum effect of at least 1.4 regardless of the quantum simulation method or the water model employed. The small quantum effect in our model is a result of two competing phenomena. Intermolecular zero point energy and tunneling effects destabilize the hydrogen-bonding network, leading to a less viscous liquid with a larger diffusion coefficient. However, this is offset by intramolecular zero point motion, which changes the average water monomer geometry resulting in a larger dipole moment, stronger intermolecular interactions, and a slower diffusion. We end by suggesting, on the basis of simulations of other potential energy models, that the small quantum effect we find in the diffusion coefficient is associated with the ability of our model to produce a single broad O-H stretching band in the infrared absorption spectrum. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3167790]