Center for Molecular Modeling (CMM), Ghent University, Technologiepark 46, 9052 Zwijnaarde, Belgium.
J Chem Phys. 2019 Mar 7;150(9):094503. doi: 10.1063/1.5085649.
In this article, we investigate the influence of anharmonicities and nuclear quantum effects (NQEs) in modelling the structural properties and thermal expansion of the empty MOF-5 metal-organic framework. To introduce NQEs in classical molecular dynamics simulations, two different methodologies are considered, comparing the approximate, but computationally cheap, method of generalised Langevin equation thermostatting to the more advanced, computationally demanding path integral molecular dynamics technique. For both methodologies, similar results were obtained for all the properties under investigation. The structural properties of MOF-5, probed by means of radial distribution functions (RDFs), show some distinct differences with respect to a classical description. Besides a broadening of the RDF peaks under the influence of quantum fluctuations, a different temperature dependence is also observed due to a dominant zero-point energy (ZPE) contribution. For the thermal expansion of MOF-5, by contrast, NQEs appear to be only of secondary importance with respect to an adequate modelling of the anharmonicities of the potential energy surface (PES), as demonstrated by the use of two differently parametrised force fields. Despite the small effect in the temperature dependence of the volume of MOF-5, NQEs do however significantly affect the absolute volume of MOF-5, in which the ZPE resulting from the intertwining of NQEs and anharmonicities plays a crucial role. A sufficiently accurate description of the PES is therefore prerequisite when modelling NQEs.
在本文中，我们研究了非谐性和核量子效应（NQEs）在模拟空 MOF-5 金属有机骨架的结构性质和热膨胀中的影响。为了在经典分子动力学模拟中引入 NQEs，我们考虑了两种不同的方法，比较了广义朗之万方程（GLE）恒温器的近似但计算成本低的方法与更先进、计算要求更高的路径积分分子动力学技术。对于这两种方法，对于所有研究的性质，都得到了相似的结果。通过径向分布函数（RDFs）探测的 MOF-5 的结构性质与经典描述有一些明显的区别。除了由于量子涨落的影响导致 RDF 峰展宽外，由于零点能（ZPE）的主导贡献，还观察到不同的温度依赖性。相比之下，对于 MOF-5 的热膨胀，NQEs 似乎相对于势能面（PES）的非谐性的适当建模只是次要的，这可以通过使用两个不同参数化的力场来证明。尽管 NQEs 对 MOF-5 体积的温度依赖性影响较小，但它们确实会显著影响 MOF-5 的绝对体积，其中 NQEs 和非谐性交织产生的 ZPE 起着至关重要的作用。因此，在建模 NQEs 时，对 PES 的充分准确描述是先决条件。
