  We consider heat transport across a harmonic chain connected at its two ends
to white-noise Langevin reservoirs at different temperatures. In the steady
state of this system the heat $Q$ flowing from one reservoir into the system in
a finite time $\tau$ has a distribution $P(Q,\tau)$. We study the large time
form of the corresponding moment generating function $<e^{-\lambda Q}>\sim
g(\lambda) e^{\tau\mu (\lambda)}$. Exact formal expressions, in terms of phonon
Green's functions, are obtained for both $\mu(\lambda)$ and also the lowest
order correction $g(\lambda)$. We point out that, in general a knowledge of
both $\mu(\lambda)$ and $g(\lambda)$ is required for finding the large
deviation function associated with $P(Q,\tau)$. The function $\mu(\lambda)$ is
known to be the largest eigenvector of an appropriate Fokker-Planck type
operator and our method also gives the corresponding eigenvector exactly.
