  In Phys. Rev. Lett. 84, 4204 (2000) (cond-mat/9905379), Kato et al. presented
quantum Monte Carlo results indicating that the critical concentration of
random non-magnetic sites in the two-dimensional antiferromagnetic Heisenberg
model equals the classical percolation density; pc=0.407254. The data also
suggested a surprising dependence of the critical exponents on the spin S of
the magnetic sites, with a gradual approach to the classical percolation
exponents as S goes to infinity. I here argue that the exponents in fact are
S-independent and equal to those of classical percolation. The apparent
S-dependent behavior found by Kato et al. is due to temperature effects in the
simulations as well as a quantum effect that masks the true asymptotic scaling
behavior for small lattices.
