  The coupled Dyson-Schwinger equations for the gluon and ghost propagators are
investigated in the Landau gauge using a two-loop improved truncation that
preserves the multiplicative renormalizability of the propagators. In this
truncation all diagrams contribute to the leading order infrared analysis. The
infrared contributions of the nonperturbative two-loop diagrams to the gluon
vacuum polarization are computed analytically, and this reveals that infrared
power behaved propagator solutions only exist when the squint diagram
contribution is taken into account. For small momenta the gluon and ghost
dressing functions behave respectively like (p^2)^{2\kappa} and
(p^2)^{-\kappa}, and the running coupling exhibits a fixed point. The values of
the infrared exponent and fixed point depend on the precise details of the
truncation. The coupled ghost-gluon system is solved numerically for all
momenta, and the solutions have infrared behaviors consistent with the
predictions of the infrared analysis. For truncation parameters chosen such
that \kappa=0.5, the two-loop improved truncation is able to produce solutions
for the propagators and running coupling which are in very good agreement with
recent lattice simulations.
