Optic property calculations of 1T-TiSe2 under CDW transition*

1T-TiSe2 shows an "unusual" changing trend of resistivity vs. temperatures. It has a lattice instability (CDW phase transition) about 200 K. Intercalating Cu into 1T-TiSe2 results in superconductivity. It was suggested that both the long-range and the short-range CDW orders exist in 1T-TiSe2, showing a pseudogap phase even above the CDW transition temperature. It was suggested that after CDW transition, 1T-TiSe2 enters in an exciton insulator state and has a bulk gap about 0.15 eV. It shows an enhanced plasmon peak at low temperatures, indicating the exciton condensate and reduced scattering rate. Due to the many-body nature of the interactions involved, it is challenging for DFT to accurately calculate the electronic structure of 1T-TiSe2 under its unconventional CDW phase. Advanced TDDFT methods with sophisticated kernel can very reasonably predict the CDW electron density for uniform electron gas, hence may be an excellent tool for plasmon related calculations.