A determination of the low energy parameters of the 2-d Heisenberg   antiferromagnet
  We perform numerical simulations of the 2-d antiferromagnetic quantum Heisenberg model using an efficient cluster algorithm. Comparing the finite size and finite temperature effects of various quantities with recent results from chiral perturbation theory we are able to determine the low energy parameters of the system very precisely. We find $e_0 = - 0.6693(1) J/a^2$ for the ground state energy density, ${\cal M}_s = 0.3074(4)/a^2$ for the staggered magnetization, $\hbar c = 1.68(1) J a$ for the spin wave velocity and $\rho_s = 0.186(4) J$ for the spin stiffness. Our results agree with experimental data for the undoped precursor insulators of high-$T_c$ superconductors. 
U.-J. Wiese and H.-P. Ying, (Bern University)