  We develop a computational approach for calculating the optical conductivity
in the augmented plane wave basis set of Wien2K and apply it for thoroughly
comparing the full dipole matrix element calculation and the Peierls
approximation. The results for SrVO3 and V2O3 show that the Peierls
approximation, which is commonly used in model calculations, works well for
optical transitions between the d orbitals. In a typical transition metal
oxide, these transitions are solely responsible for the optical conductivity at
low frequencies. The Peierls approximation does not work, on the other hand,
for optical transitions between p- and d-orbitals which usually became
important at frequencies of a few eVs
