Excluded volume effects for frequency moments of the spin autocorrelation function of the Heisenberg model on a square lattice at high temperatures

The expansion coefficients in powers of time (or frequency moments) of the spill autocorrelation Function are represented at the simple self-consisten t approximation as a sum of weighted trees on a Bethe lattice. Using the co mputer numeration and the Monte Carlo method for self-avoidingly embedding these trees on the square lattice, we estimate the moments and the converge nce radius of the expansion. We show that the moments decrease and the radi us increases in consequence of the volume exclusion.