Tao Jianmin, Yang Jing, Rappe Andrew M
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
J Chem Phys. 2015 Apr 28;142(16):164302. doi: 10.1063/1.4918761.
Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C60). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.
升华能是分子晶体最重要的性质之一，但由于长程范德华（vdW）吸引相互作用起着重要作用，所以很难进行研究。在此，我们应用高效的半局域密度泛函理论（DFT），并结合动态屏蔽vdW相互作用（DFT + vdW）、罗格斯 - 查尔姆斯非局域vdW - DF以及由格林等人开发的基于对相互作用的色散校正DFT - D2，来研究富勒烯的升华。我们发现，由于富勒烯之间轨道重叠引起的相互作用的短程部分小到可以忽略不计。我们的计算表明，富勒烯价电子对vdW相互作用存在很强的屏蔽效应。另一方面，高阶贡献可能与主导项一样重要。原因如下：（i）富勒烯分子表面是金属性的，因此具有很高的极化率；（ii）富勒烯固体的带隙很小（小于2 eV）；（iii）固相中的富勒烯分子堆积非常密集，导致高价电子密度和较小的平衡分子间距离（对于C60，第一近邻距离仅约为10 Å）。然而，这两种效应的贡献相反，导致这两种贡献之间有显著的误差抵消。我们证明，通过考虑高阶贡献和动态屏蔽，DFT + vdW方法可以得到与参考值高度吻合的富勒烯升华能，其次是vdW - DF和DFT - D2。这项研究所得出的见解对于更好地理解纳米结构固体中vdW相互作用的长程性质很重要。
