  Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of
in-plane couplings J_1 + J_2 = 9.5 \pm 1.5 K and the inter-plane coupling
J_{perp} \sim 0.2 - 0.3 K agree with recent experimental data, the ratio
J_2/J_1 \sim 12 exceeds the reported value by an order of magnitude. Using
geometrical considerations, high temperature expansions and perturbative mean
field theory, we show that the LDA derived exchange constants lead to a
remarkably accurate description of the properties of these materials including
specific heat, susceptibility, Neel temperature and NMR spectra.
