The paper presents a method for calculating the linear optical properties of solids using the full-potential linearized augmented plane wave (LAPW) method. The authors derive the dielectric tensor within the random-phase approximation (RPA), considering both interband and intraband contributions. They evaluate the momentum matrix elements for the LAPW basis and present results for aluminum and gold, cross-checking their findings with sum rules. The study highlights the sensitivity of optical spectra to Brillouin zone sampling and investigates the influence of relativistic effects on the dielectric function, finding that scalar-relativistic effects are more significant than spin-orbit coupling. The interpretability of Kohn-Sham eigenstates in terms of excited states is also discussed. The paper provides a detailed theoretical background, including symmetry considerations and Kramers-Kronig relations, and discusses the convergence of the optical spectra with respect to the k-point mesh.The paper presents a method for calculating the linear optical properties of solids using the full-potential linearized augmented plane wave (LAPW) method. The authors derive the dielectric tensor within the random-phase approximation (RPA), considering both interband and intraband contributions. They evaluate the momentum matrix elements for the LAPW basis and present results for aluminum and gold, cross-checking their findings with sum rules. The study highlights the sensitivity of optical spectra to Brillouin zone sampling and investigates the influence of relativistic effects on the dielectric function, finding that scalar-relativistic effects are more significant than spin-orbit coupling. The interpretability of Kohn-Sham eigenstates in terms of excited states is also discussed. The paper provides a detailed theoretical background, including symmetry considerations and Kramers-Kronig relations, and discusses the convergence of the optical spectra with respect to the k-point mesh.