  We explore a formulation of the S-matrix in terms of the path integral with
specified asymptotic data, as originally proposed by Arefeva, Faddeev, and
Slavnov. In the tree approximation the S-matrix is equal to the exponential of
the classical action evaluated on-shell. This formulation is well-suited to
questions involving asymptotic symmetries, as it avoids reference to
non-gauge/diffeomorphism invariant bulk correlators or sources at intermediate
stages. We show that the soft photon theorem, originally derived by Weinberg
and more recently connected to asymptotic symmetries by Strominger and
collaborators, follows rather simply from invariance of the action under large
gauge transformations applied to the asymptotic data. We also show that this
formalism allows for efficient computation of the S-matrix in curved spacetime,
including particle production due to a time dependent metric.
