Computational phase diagrams of noble gas hydrates under pressure

We present results from a first-principles study on the stability of noble gas-water compounds in the pressure range 0-100 kbar. Filled-ice structures based on the host water networks ice-Ih, ice-Ic, ice-II, and C0 interacting with guest species He, Ne, and Ar are investigated, using density functional theory (DFT) with four different exchange-correlation functionals that include dispersion effects to various degrees: the non-local density-based optPBE-van derWaals (vdW) and rPW86-vdW2 functionals, the semi-empirical D2 atom pair correction, and the semi-local PBE functional. In the He-water system, the sequence of stable phases closely matches that seen in the hydrogen hydrates, a guest species of comparable size. In the Ne-water system, we predict a novel hydrate structure based on the C0 water network to be stable or at least competitive at relatively low pressure. In the Ar-water system, as expected, no filled-ice phases are stable; however, a partially occupied Ar-C0 hydrate structure is metastable with respect to the constituents. The ability of the different DFT functionals to describe the weak host-guest interactions is analysed and compared to coupled cluster results on gas phase systems.

本研究通过第一性原理计算，探究了0~100千巴（kbar）压力区间内稀有气体-水化合物的稳定性。本研究采用密度泛函理论（DFT），选取四类不同程度考虑色散效应的交换-关联泛函，对以冰-Ih、冰-Ic、冰-II及C0型水网络为主体框架、与氦（He）、氖（Ne）、氩（Ar）客体粒子相互作用的填充冰结构开展了系统研究，所用泛函包括：基于非局域密度的optPBE-范德华（vdW）泛函、rPW86-vdW2泛函、半经验D2原子对校正方法，以及半局域PBE泛函。
在氦-水体系中，稳定相的演化序列与尺寸相近的客体粒子所形成的氢气水合物高度吻合。在氖-水体系中，本研究预测基于C0型水网络的新型水合物结构在较低压力区间内具备稳定性，或至少具有竞争优势。在氩-水体系中，正如预期，无填充冰结构可稳定存在；但相较于其组分而言，部分占据位点的氩-C0型水合物结构呈亚稳态。本研究还分析了不同DFT泛函对弱主体-客体相互作用的描述能力，并将其与气相体系的耦合簇计算结果进行了对比。