DYNAMIC CORRELATIONS IN A CLASSICAL 2-DIMENSIONAL HEISENBERG-ANTIFERROMAGNET - COMMENT

The derivation by Wysin and Bishop [Phys. Rev. B 42, 810 (1990)] of a general result for the dynamic structure function, S(q,t), of classical isotropic Heisenberg systems is shown to be incorrect. A correct quantum-mechanical proof of a general theorem, which is valid for all translationally invariant systems and which produces the result of Wysin and Bishop as a special classical case, is provided. It is also pointed out that another condition on S(q,t) for rotationally invariant systems such as the isotropic Heisenberg model does not appear to be satisfied by the results of Wysin and Bishop.