Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Chem Phys. 2012 Sep 14;137(10):104304. doi: 10.1063/1.4746157.
Using a newly developed and recently parameterized classical empirical simulation model for water that involves explicit three-body interactions, we determine the eleven most stable isomers of the water hexamer. We find that the lowest energy isomer is one of the cage structures, in agreement with far-IR and microwave experiments. The energy ordering for the binding energies is cage > glove > book > bag > chair > boat > chaise, and energies relative to the cage are in good agreement with CCSD(T) calculations. The three-body contributions to the cage, book, and chair are also in reasonable agreement with CCSD(T) results. The energy of each isomer results from a delicate balance involving the number of hydrogen bonds, the strain of these hydrogen bonds, and cooperative and anti-cooperative three-body interactions, whose contribution we can understand simply from the form of the three-body interactions in the simulation model. Oxygen-oxygen distances in the cage and book isomers are in good agreement with microwave experiments. Hydrogen-bond distances depend on both donor and acceptor, which can again be understood from the three-body model. Fully anharmonic OH-stretch spectra are calculated for these low-energy structures, and compared with shifted harmonic results from ab initio and density functional theory calculations. Replica-exchange molecular dynamics simulations were performed from 40 to 194 K, which show that the cage isomer has the lowest free energy from 0 to 70 K, and the book isomer has the lowest free energy from 70 to 194 K. OH-stretch spectra were calculated between 40 and 194 K, and results at 40, 63, and 79 K were compared to recent experiments, leading to re-assignment of the peaks in the experimental spectra. We calculate local OH-stretch cumulative spectral densities for different donor-acceptor types and compare to analogous results for liquid water.
利用最近开发的涉及明确三体相互作用的新型经典经验模拟模型，我们确定了水六聚体的 11 个最稳定异构体。我们发现最低能量异构体是笼状结构之一，与远红外和微波实验一致。结合能的能量排序为笼状>手套状>书本状>袋状>椅状>船状>躺椅状，并且相对于笼状的能量与 CCSD(T)计算结果非常吻合。三体对笼状、书本状和椅状的贡献也与 CCSD(T)结果基本一致。每个异构体的能量来自于氢键数量、这些氢键的应变以及协同和反协同三体相互作用之间的微妙平衡，我们可以简单地从模拟模型中的三体相互作用形式理解它们的贡献。笼状和书本状异构体中的氧-氧距离与微波实验吻合良好。氢键距离取决于供体和受体，这也可以从三体模型中理解。对这些低能结构计算了完全非谐 OH 伸缩光谱，并与从头算和密度泛函理论计算的位移谐波结果进行了比较。从 40 到 194 K 进行了 replica-exchange 分子动力学模拟，结果表明从 0 到 70 K 时笼状异构体具有最低自由能，从 70 到 194 K 时书本状异构体具有最低自由能。在 40 到 194 K 之间计算了 OH 伸缩光谱，并将 40、63 和 79 K 的结果与最近的实验进行了比较，导致实验光谱中峰的重新分配。我们计算了不同供体-受体类型的局部 OH 伸缩累积光谱密度，并与液态水的类似结果进行了比较。
