  Consider the heat kernel $p(t,x,y)$ on the universal cover $X$ of a
Riemannian manifold $M$ of negative curvature. We show the local limit theorem
for $p$ : $$\lim_{t \to \infty} t^{3/2}e^{\lambda_0 t} p(t,x,y)=C(x,y),$$ where
$\lambda_0$ is the bottom of the spectrum of the geometric Laplacian and
$C(x,y)$ is a positive function which depends on $x, y \in X$.
  We also show that the $\lambda_0$-Martin boundary of $X$ is equal to its
topological boundary. The Martin decomposition of $C(x,y)$ gives a family of
measures $\{\mu^{\lambda_0}_x \}$ on $\partial \widetilde{M}$.
  We show that $\{\mu^{\lambda_0}_x \}$ is the unique family minimizing the
energy or the Rayleigh quotient of Mohsen.
  We use the uniform Harnack inequality on the boundary $\partial X$ and the
uniform three-mixing of the geodesic flow on the unit tangent bundle $SM$ for
suitable Gibbs-Margulis measures.
