Microcanonical simulation of the four-dimensional SU(2) non-linear sigma model

We numerically study the four-dimensional SU(2) non-linear sigma model in the microcanonical ensemble by a method that directly enumerates the partition function. We studied lattices of sizes up to 84 and were able to compute the critical coupling to good accuracy and estimate the exponent ν. The method we describe can easily be extended to study theories where the measure is non-positive definite such as sigma models with a Wess-Zumino-Witten topological term. (orig.)