  The partition function of a quantum statistical system in flat space can
always be written as an integral over a classical Boltzmann factor $\exp[
-\beta V^{\rm eff cl({\bf x}_0)]$, where $V^{\rm eff cl({\bf x}_0)$ is the
so-called effective classical potential containing the effects of all quantum
fluctuations. The variable of integration is the temporal path average ${\bf
x_0\equiv \beta ^{-1}\int_0^ \beta d\tau {\bf x}(\tau)$. We show how to
generalize this concept to paths $q^\mu(\tau)$ in curved space with metric
$g_{\mu \nu (q)$, and calculate perturbatively the high-temperature expansion
of $V^{\rm eff cl(q_0)$. The requirement of independence under coordinate
transformations $q^\mu(\tau)\to q'^\mu(\tau)$ introduces subtleties in the
definition and treatment of the path average $q_0^\mu$, and covariance is
achieved only with the help of a suitable Faddeev-Popov procedure.
