We present a first-principles characterization of the excitation spectra of the representative transition metal dichalcogenide compound, $1T\text{−}{\mathrm{TiSe}}_2$, realized in the time-dependent density functional theory (TDDFT) framework. We discuss the origin and physical nature of different collective electronic excitations present in the spectra in the energy region below 30 eV in terms of interband transitions. We have found three kinds of electronic modes in agreement with experimental findings obtained in electron energy-loss spectroscopy and ellipsometry experiments. For the small in-plane momenta, these three modes interpreted as bulk $\sigma +\pi , \pi$, and interband plasmons are characterized by energies of 19.9, 6.7, and 2.3 eV, respectively. Along the out-of-plane axis, the energy of the bulk $\sigma +\pi$ plasmon is 20.6 eV. The $\pi$ plasmon is found at energies around 10 eV. We follow the dispersion of all these modes over an extended momentum range. Our calculations confirm the absence of the interband plasmon along the out-of-plane direction in the lower-energy part.

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• Charge dynamics
• Dielectric properties
• Excitons
• Transition metal dichalcogenides
• Electron energy loss spectroscopy
• Time-dependent DFT