Van der Waals (vdW) solids have attracted great attention ever since the discovery of graphene, with the essential feature being the weak chemical bonding across the vdW gap. The nature of these weak interactions is decisive for many extraordinary properties, but it is a strong challenge for current theory to accurately model long-range electron correlations. Here we use synchrotron X-ray diffraction data to precisely determine the electron density in the archetypal vdW solid, TiS₂, and compare the results with density functional theory calculations. Quantitative agreement is observed for the chemical bonding description in the covalent TiS₂ slabs, but significant differences are identified for the interactions across the gap, with experiment revealing more electron deformation than theory. The present data provide an experimental benchmark for testing theoretical models of weak chemical bonding.

自从发现石墨烯以来，范德华（vdW）固体就引起了极大的关注，其基本特征是跨越 vdW 间隙的弱化学键合。这些弱相互作用的性质对于许多非凡的性质是决定性的，但对于当前理论来说，准确地模拟远程电子相关性是一个巨大的挑战。在这里，我们使用同步加速器 X 射线衍射数据来精确确定原型 vdW 固体 TiS _{2 中}的电子密度，并将结果与​​密度泛函理论计算进行比较。观察到共价 TiS _{2 中}化学键合描述的定量一致性板，但发现间隙之间的相互作用存在显着差异，实验表明电子变形比理论更多。目前的数据为测试弱化学键合的理论模型提供了实验基准。