  We apply optimized perturbation theory to the quark-meson model at finite
temperature T and quark chemical potential mu. The effective potential is
calculated to one loop both in the chiral limit and at the physical point and
used to study the chiral dynamics of two-flavor QCD. The critical temperature
and the order of the phase transition depends heavily on whether or not one
includes the bosonic and fermionic vacuum fluctuations in the effective
potential. A full one-loop calculation in the chiral limit predicts a
first-order transition for all values of mu. At the physical point, one finds a
crossover in the whole $\mu-T$ plane.
