  We use recent lattice data on the gluon and ghost propagators, as well as the
Kugo-Ojima function, in order to extract the non-perturbative behavior of two
particular definitions of the QCD effective charge, one based on the pinch
technique construction, and one obtained from the standard ghost-gluon vertex.
The construction relies crucially on the definition of two dimensionful
quantities, which are invariant under the renormalization group, and are built
out of very particular combinations of the aforementioned Green's functions.
The main non-perturbative feature of both effective charges, encoded in the
infrared finiteness of the gluon propagator and ghost dressing function used in
their definition, is the freezing at a common finite (non-vanishing) value, in
agreement with a plethora of theoretical and phenomenological expectations. We
discuss the sizable discrepancy between the freezing values obtained from the
present lattice analysis and the corresponding estimates derived from several
phenomenological studies, and attribute its origin to the difference in the
gauges employed. A particular toy calculation suggests that the modifications
induced to the non-perturbative gluon propagator by the gauge choice may indeed
account for the observed deviation of the freezing values.
