First-principle study of a vdW material: TiS2

Van der Waals (vdW) solids have attracted great attention since the discovery of graphene, with the essential feature being the weak interlayer bonding. 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. Nowadays, Titanium disulfide, TiS2 , which is an archetypal vdW solid, is receiving considerable attention. In fact, recent experimental analysis of chemical bonding in TiS2 , based on synchrotron X-ray diffraction data, allowed to obtain an accurate profile of the electron density in this material. Comparison with available theoretical electron-density calculations, based on the Density Func- tional Theory (DFT), shows that, while a quantitative agreement is observed for the chemical bonding description in the covalent TiS2 slabs, significant differences are instead identified for the interlayer interactions. In fact, the experiments reveal more electron deformation than theory, thus suggesting than the interlayer interactions are significantly stronger than the current theo- retical description predicts. In order to shed light on this discrepancy and to obtain a complete theoretical description of TiS2 , the Quantum Espresso first principles package, based on the DFT, is applied, by adopting various theoretical schemes to consider, at different levels, the vdW interactions also including the most recent developments. Several first principles simulations are carried out to evaluate structural, electronic and energetic properties of the system. In particular, we obtain the electron density distributions which can be directly compared with experimental data. Careful data analysis allows to get a better knowledge of TiS2 properties and also to assess the quality of the newest theoretical approaches. Employing some of the most popular DFT functionals, we reach reasonably satisfactory agree- ments with experimental data, both for structural and electronic properties. Moreover, we propose a tuning of the q parameter in the original vdW-DF functional of Hamada, obtaining a value of the charge density at the bond critical point in the S-S interlayer fragment, in better agreement with experimental data.