Before revision: We study the computational complexity of approximating the partition function of the ferromagnetic Ising model in the Lee-Yang circle of zeros given by |\lambda|=1, where \lambda is the external field of the model . Complex-valued parameters for the Ising model are relevant for quantum circuit computations and phase transitions in statistical physics, but have also been key in the recent deterministic approximation scheme for all |\lambda|\neq 1 by Liu, Sinclair, and Srivastava.
After revision: We study the computational complexity of approximating the partition function of the ferromagnetic Ising model with the external field parameter \lambda on the unit circle in the complex plane . Complex-valued parameters for the Ising model are relevant for quantum circuit computations and phase transitions in statistical physics, but have also been key in the recent deterministic approximation scheme for all |\lambda|\neq 1 by Liu, Sinclair, and Srivastava.