Spin correlations of 2D quantum antiferromagnet at low temperatures and a direct comparison with neutron-scattering experiments.

Correlation functions of the spin-1/2 2D Heisenberg antiferromagnet at low temperatures are computed via a large-scale Monte Carlo simulation on 128{times}128 lattices. The correlation length is found to be accurately described by the exponentially divergent form typical of classical Heisenberg spins in two dimensions. The large correlation lengths measured are directly compared with the neutron-scattering experiments on La{sub 2}CuO{sub 4}. The excellent fit provides a first-principles determination of the exchange coupling: {ital J}=1450 {plus minus}30 K.