First-principles calculation of vibrational Raman spectra of tetrahedral amorphous carbon

The nonresonant vibrational Raman spectra of tetrahedral amorphous carbon are calculated from first principles. The structural model was generated using Car-Parinello molecular dynamics, the vibrational modes are determined using the linear response approach and Raman tensors are calculated using the finite electric field method. Our theoretical visible and reduced Raman spectra show an overall good agreement with experimental spectra, and better than previous calculated results. The analysis in terms of atomic vibrations shows that the Raman spectrum mainly comes from sp² contribution, G peak is due to the stretching vibration of any pair of sp² atoms and only a small sp³ contribution can be noticed. The differences between peak intensities of reduced theoretical and experimental results mainly come from defects and the high sp³ content in our simulated structure.