Periodic Hartree–Fock methods were used to calculate the geometric and electronic properties of 2H-MoS2 , 1T-MoS2, the (100) surface of MoS2 and Li adsorbed thereon. For the calculations, the structures were generated by an extension of unit cells to the respective bulk structures (1T- and 2H-MoS2) or by cutting sections, each consisting of six or eight layers of sulfur and molybdenum, from a crystal ((100) surface of MoS2 with and without adsorbed Li). Structural optimization was performed with a post Hartree–Fock DFT correlation correction. The calculated structures of 2H-MoS2 and 1T-MoS2 are in good agreement with experimental data and the metastable and metallic properties of 1T-MoS2 are also described correctly. The relaxation of the (100) surface of 2H-MoS2 leads to a minor reconstruction of the surface accompanied by the formation of S2 species and an inward relaxation of Mo atoms. Adsorption of Li on this surface is favoured in the high symmetry positions above the van der Waals gap. Relaxation of the Li-covered (100) surface of 2H-MoS2 shifts the Li atoms towards the S2 pairs and closer to the surface. Upon adsorption, the system becomes metallic and delocalized surface states form at the Fermi level due to electron transfer processes from the Li atoms to the surface layers of MoS2.