  We present a comprehensive study of plasmon dispersions in simple metals and
Heusler compounds based on an accurate ab-initio evaluation of the momentum and
frequency dependent dielectric function in the random-phase approximation.
Using a momentum-dependent tetrahedron method for the computation of the
dielectric function, we extract and analyze "full" and "intraband" plasmon
dispersions: The "full" plasma dispersion is obtained by including all bands,
the intraband plasma dispersion by including only intraband transitions. For
the simple metals silver and alu- minum, we show that the intraband plasmon
dispersion has an unexpected downward slope and is therefore markedly different
from the results of an effective-mass electron-gas model and the full plasmon
dispersion. For the two Heusler compounds Co2FeSi and Co2MnSi, we present
spectra for the dielectric function, their loss functions and plasmon
dispersions. The latter exhibit the same negative intraband plasmon dispersion
as found in the simple metals. We also discuss the influence of spin-mixing on
the plasmon dispersion.
