  Single-ion magnetic anisotropy in molecular magnets leads to spin flip
excitations that can be measured by inelastic scanning tunneling microscope
(STM) spectroscopy. Here I present a semi ab initio scheme to compute the
spectral features associated with inelastic spin flip excitations and Kondo
effect of single molecular magnets. To this end density functional theory
calculations of the molecule on the substrate are combined with more
sophisticated many-body techniques for solving the Anderson impurity problem of
the spin-carrying orbitals of the magnetic molecule coupled to the rest of the
system, containing a phenomenological magnetic anisotropy term. For calculating
the STM spectra an exact expression for the dI/dV in the ideal STM limit, when
the coupling to the STM tip becomes negligibly small, is derived. In this limit
the dI/dV is simply related to the spectral function of the molecule-substrate
system. For the case of an Fe porphyrin molecule on the Au(111) substrate, the
calculated STM spectra are in good agreement with recently measured STM
spectra, showing the typical step features at finite bias associated with spin
flip excitation of a spin-1 quantum magnet. For the case of Kondo effect in Mn
porphyrin on Au(111), the agreement with the experimental spectra is not as
good due to the neglect of quantum interference in the tunneling.
