  By analytic continuation to real theta of data obtained from numerical
simulation at imaginary theta we study the Haldane conjecture and show that the
O(3) non-linear sigma model with a theta term in 2 dimensions becomes massless
at theta=3.10(5). A modified cluster algorithm has been introduced to simulate
the model with imaginary theta. Two different definitions of the topological
charge on the lattice have been used; one of them needs renormalization to
match the continuum operator. Our work also offers a successful test for
numerical methods based on analytic continuation.
