New method for calculating transport coefficients and application to the Heisenberg chain

Abstract
In this article, a new simple method to calculate memory functions within the Mori formalism is presented. It is then used to derive the relaxation function for a Heisenberg chain. The results for different choices of relevant variables are compared with experiments and other theories. It is found that for the right choice of variables, the results are in much better agreement with recent experiments on tetramethylammonium-manganese-trichloride (TMMC) than the results of other theories.