  We describe the software package SPEX, which allows first-principles
calculations of quasiparticle and collective electronic excitations in solids
using techniques from many-body perturbation theory. The implementation is
based on the full-potential linearized augmented-plane-wave (FLAPW) method,
which treats core and valence electrons on an equal footing and can be applied
to a wide range of materials, including transition metals and rare earths.
After a discussion of essential features that contribute to the high numerical
efficiency of the code, we present illustrative results for quasiparticle band
structures calculated within the GW approximation for the electronic
self-energy, electron-energy-loss spectra with inter- and intraband transitions
as well as local-field effects, and spin-wave spectra of itinerant
ferromagnets. In all cases the inclusion of many-body correlation terms leads
to very good quantitative agreement with experimental spectroscopies.
