We find the corrections to the conventional 1/S expansion for the spin-wave velocity in the two-dimensional S=1/2 Heisenberg antiferromagnet. We do this by solving exactly for the ground and excited states of small clusters of up to 25 spins. We use open boundary conditions with a staggered Weiss effective field consistent with the correct quantum Monte Carlo staggered magnetization ${\mathit{M}}_{\mathit{S}}$=0.31. The numerical results are directly compared with the 1/S expansion for the same clusters. The exact spin-wave velocity is given by c=Z1.158 √2 Ja, where Z is within 2% of unity. Our best estimate gives Z≊1.006.

• Received 4 April 1990

DOI:https://doi.org/10.1103/PhysRevB.42.4208