  We point out a close connection between the Moser-Tardos algorithmic version
of the Lov\'asz Local Lemma, a central tool in probabilistic combinatorics, and
the cluster expansion of the hard core lattice gas in statistical mechanics. We
show that the notion of witness trees given by Moser and Tardos is essentially
coincident with that of Penrose trees in the Cluster expansion scheme of the
hard core gas. Such an identification implies that the Moser Tardos algorithm
is successful in a polynomial time if the Cluster expansion converges.
