  We consider the magnetic interaction of manganese phtalocyanine (MnPc)
absorbed on Pb layers which were grown on a Si substrate. We perform an
ab-initio calculation of the density of states and Kondo temperature as
function of the number of Pb monolayers. Comparison to experimental data [Phys.
Rev. Lett. 99, 256601 (2007)] then allows us to determine the exchange coupling
constant J between the spins of the adsorbed molecules and those of the Pb
host. This approach gives rise to a general and reliable method for obtaining
J, by combining experimental and numerical results.
