We present a method for calculating dielectric matrices of periodic systems. Unlike the conventional method, which uses a plane-wave basis, the present method employs a product basis, which, in the linear-muffin-tin-orbital formalism, consists of products of orbitals. The method can be used for any system, including sp as well as narrow band systems. We demonstrate the applicability of our method by calculating the energy-loss spectra of Ni and Si, including local-field effects that require the full dielectric matrix. Good agreement with experiment is found. The small number of basis functions makes the method suitable for self-energy calculations within the GW approximation, without making the so-called plasmon-pole approximation for the dielectric matrix.

• Received 24 February 1994

DOI:https://doi.org/10.1103/PhysRevB.49.16214