  We calculated the optical response of metallic and insulating VO2 using the
LDA approach. The band structure calculation was based in the full-potential
linear-muffin-tin method. The imaginary part of the dielectric function e2(w)
is related to the different optical transitions. The Drude tail in the
calculation of the metallic phase corresponds to intra-band d-d transitions.
The calculation in the insulating phase is characterized by the transitions to
the d||* band. The low frequency features, 0.0-5.0 eV, correspond to V 3d-V 3d
transitions, whereas the high frequency structures, 5.0-12 eV, are related to O
2p-V 3d transitions. The calculation helps to explain the imaginary part of the
dielectric function e2(w), as well as the electron-energy-loss and reflectance
spectra. The results reproduce not only the energy position and relative
intensity of the features in the spectra, but also the main changes across the
metal-insulator transition and the polarization dependence. The main difference
is a shift of about 0.6 eV in the calculation of the insulating phase. This
discrepancy arises because the LDA calculation underestimates the value of the
band gap.
