Cutini Michele, Maschio Lorenzo, Ugliengo Piero
Department of Chemistry and NIS (Nanostructured Interfaces and Surfaces) Center, University of Turin, Via P. Giuria 5-7, 10125 Turin, Italy.
J Chem Theory Comput. 2020 Aug 11;16(8):5244-5252. doi: 10.1021/acs.jctc.0c00149. Epub 2020 Jul 17.
In this work, we have computed the exfoliation energy (the energy required to separate a single layer from the bulk structure), the interlayer distance, and the thermodynamic state functions for representative layered inorganic minerals such as Brucite, Portlandite, and Kaolinite, while leaving the more classical 2D transition-metal dichalcogenides (like MoS) for future work. Such materials are interesting for several applications in the field of adsorption and in prebiotic chemistry. Their peculiar features are directly controlled by the exfoliation energy. In materials without cations/anions linking different layers, the interactions keeping the layers together are of weak nature, mainly dispersion London interactions and hydrogen bonds, somehow challenging to deal with computationally. We used Hartree-Fock (HF) and density functional theory (DFT) approaches focusing on the role of dispersion forces using the popular and widespread Grimme's pairwise dispersion schemes (-D2 and -D3) and, as a reference method, the periodic MP2 approach based on localized orbitals (LMP2). The results are highly dependent on the choice of the scheme adopted to account for dispersion interactions. D2 and D3 schemes combined with either HF or DFT lead to overestimated exfoliation energies (about 2.5 and 1.7 times higher than LMP2 data for Brucite/Portlandite and Kaolinite) and underestimated interlayer distances (by about 3.5% for Brucite/Portlandite). The reason is that D2 and D3 corrections are based on neutral atomic parameters for each chemical element which, instead, behave as cations in the considered layered material (Mg, Ca, and Al), causing overattractive interaction between layers. More sophisticated dispersion corrections methods, like those based on nonlocal vdW functionals, many body dispersion model, and exchange-hole dipole moment not relying on atom-typing, are, in principle, better suited to describe the London interaction of ionic species. Nonetheless, we demonstrate that good results can be achieved also within the simpler D2 and D3 schemes, in agreement with previous literature suggestions, by adopting the dispersion coefficients of the preceding noble gas for the ionic species, leading to energetics in good agreement with LMP2 and structures closer to the experiments.
在这项工作中，我们计算了代表性层状无机矿物（如水镁石、氢氧钙石和高岭石）的剥离能（从块状结构分离出单层所需的能量）、层间距和热力学状态函数，而将更经典的二维过渡金属二硫属化物（如MoS）留待未来研究。这类材料在吸附领域和益生元化学中有多种应用，因而备受关注。它们的独特性质直接由剥离能控制。在没有阳离子/阴离子连接不同层的材料中，使各层保持在一起的相互作用较弱，主要是色散伦敦相互作用和氢键，这在计算处理上颇具挑战性。我们使用了Hartree - Fock（HF）和密度泛函理论（DFT）方法，采用流行且广泛应用的Grimme成对色散方案（-D2和 -D3）来关注色散力的作用，并作为参考方法，使用基于定域轨道的周期性MP2方法（LMP2）。结果高度依赖于用于考虑色散相互作用的方案选择。D2和D3方案与HF或DFT相结合会导致剥离能被高估（对于水镁石/氢氧钙石和高岭石，比LMP2数据高约2.5倍和1.7倍），层间距被低估（对于水镁石/氢氧钙石约低估3.5%）。原因是D2和D3校正基于每种化学元素的中性原子参数，但在所考虑的层状材料（Mg、Ca和Al）中这些元素表现为阳离子，导致层间相互作用过强。原则上，更复杂的色散校正方法，如基于非局域范德华泛函、多体色散模型和不依赖原子类型的交换空穴偶极矩的方法，更适合描述离子物种的伦敦相互作用。尽管如此，我们证明，按照先前文献的建议，通过采用前一个稀有气体的色散系数来处理离子物种，在更简单的D2和D3方案中也能取得良好的结果，从而得到与LMP2能量相符且结构更接近实验值的结果。
