  Let $X_t$ be the (reflecting) diffusion process generated by
$L:=\Delta+\nabla V$ on a complete connected Riemannian manifold $M$ possibly
with a boundary $\partial M$, where $V\in C^1(M)$ such that $\mu(d x):=
e^{V(x)}d x$ is a probability measure. We estimate the convergence rate for the
empirical measure $\mu_t:=\frac 1 t \int_0^t \delta_{X_s\d s$ under the
Wasserstein distance. As a typical example, when $M=\mathbb R^d$ and $V(x)=
c_1- c_2 |x|^p$ for some constants $c_1\in \mathbb R, c_2>0$ and $p>1$, the
explicit upper and lower bounds are present for the convergence rate, which are
of sharp order when either $d<\frac{4(p-1)}p$ or $d\ge 4$ and $p\to\infty$.
