Periodic Hartree-Fock study of TiS2

The structural and electronic properties of titanium disulfide were investigated at the ab initio, all-electron, periodic Hartree-Fock level, using an extended basis set and a posteriori density functional correlation corrections to the total energy. Calculated lattice parameters, bulk modulus, linear moduli, cohesive energy, elastic constants, Mulliken populations, band structure, and density of states are reported. The pressure induced semiconductor-to-semimetal phase transition in titanium disulfide is found to result from an indirect band overlap between a sulfur 3p(z)-based valence band at Gamma and a titanium Sd-based conduction band at L. The pressure shift of this overlap in the metallic phase is predicted to be 30 meV/GPa. The phase transition is predicted to occur at a pressure of 84 GPa, which is far greater than the experimental value of 4.0 +/- 0.5 GPa. This large error is due to the large overestimation of the zero-pressure optical band gap in the Hartree-Fock approximation. Correcting this energy gap by using empirical rigid-band shifts results in a predicted transition pressure between 1.5 and 4.0 GPa, which is consistent with experiment. Lattice parameters a, c, and z at the phase transition are predicted to be 5.42(3) Angstrom, 3.284(5) Angstrom and 0.2567(16), respectively.