On the behaviour of a two-dimensional Heisenberg antiferromagnet at very low temperatures

We present an analytical formula for the ratio of the physical spin correla tion length of a two-dimensional Heisenberg antiferromagnet on a square lat tice, and the one which is actually computed in numerical simulations. This latter correlation length is deduced from the second moment of the structu re factor at the antiferromagnetic momentum Q. We show that the ratio is ve ry close to one, in agreement with a previously obtained numerical result b ased on the 1/N expansion.