We present a flexible and effective ab-initio scheme to build many-body
models for molecular nanomagnets, and to calculate magnetic exchange couplings
and zero-field splittings. It is based on using localized Foster-Boys orbitals
as one-electron basis. We apply this scheme to three paradigmatic systems, the
antiferromagnetic rings Cr8 and Cr7Ni and the single molecule magnet Fe4. In
all cases we identify the essential magnetic interactions and find excellent
agreement with experiments.Comment: 5 pages, 3 figure