  We discuss QCD condensate contributions to the gluon propagator both in the
fixed-point gauge and in covariant gauges for the external QCD vacuum gluon
fields with the conclusion that a covariant gauge is essential to obtain a
gauge invariant QCD vacuum energy density difference and to retain the
unitarity of the quark scattering amplitude. The gauge-invariant QCD condensate
contributions to the effective one-gluon exchange potential are evaluated by
using the effective gluon propagator which produces a gauge-independent quark
scattering amplitude.
