  In this work we compare Monte Carlo (MC) simulations of electron transport
properties with reflection electron energy loss measurements in diamond and
graphite films. We assess the impact of different approximations of the
dielectric response on the observables of interest for the characterization of
carbon-based materials. We calculate the frequency-dependent dielectric
response and energy loss function of these materials in two ways: a full ab
initio approach, in which we carry out time-dependent density functional
simulations in linear response for different momentum transfers, and a
semi-classical model, based on the Drude--Lorentz extension to finite momenta
of the optical dielectric function. Ab initio calculated dielectric functions
lead to a better agreement with electron energy loss measured spectra with
respect the widely used Drude-Lorentz model. This discrepancy is particularly
evident for insulators and semiconductors beyond the optical limit ($\mathbf{q}
\neq 0$), where single particle excitations become relevant. Furthermore, we
show that the behaviour of the energy loss function at different accuracy
levels has a dramatic effect on other physical observables, such as the
inelastic mean free path and the stopping power in the low energy regime ($<
100$ eV) and thus on the accuracy of MC simulations.
