Energy fluctuations in a classical Heisenberg chain

The dynamic structure factor of the energy density fluctuations in the classical Heisenberg chain is studied theoretically. Using exact expressions for the first six frequency moments and the knowledge of the exact T = 0 result, simple approximations are used to calculate the spectrum. We show rigorously that the time-dependent correlation function is different for ferro- and antiferromagnetic coupling if T ≠ 0