  We study exchange couplings in the organic magnets copper(II) phthalocyanine
(Cu(II)Pc) and manganese(II) phthalocyanine (Mn(II)Pc) by a combination of
Green's function perturbation theory and \textsl{ab initio} density-functional
theory (DFT). Based on the indirect exchange model our perturbation-theory
calculation of Cu(II)Pc qualitatively agrees with the experimental
observations. DFT calculations performed on Cu(II)Pc dimer show a very good
quantitative agreement with exchange couplings that we extract by using a
global fitting for the magnetization measurements to a spin-1/2 Bonner-Fisher
model. These two methods give us remarkably consistent trends for the exchange
couplings in Cu(II)Pc when changing the stacking angles. The situation is more
complex for Mn(II)Pc owing to the competition between super-exchange and
indirect exchange.
