The quantum mechanical effective Hamiltonian of crystal field (EHCF) methodology (previously developed for describing electronic structure of transition metal complexes) is combined with the Gillespie-Kepert version of molecular mechanics (MM) in order to describe multiple potential energy surfaces (PES) of the Werner-type complexes corresponding to different spin states of the latter. The procedure thus obtained is a special version of the hybrid quantum mechanics/molecular mechanics approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving aside the effects of the d shell. The quantum mechanics part (EHCF) is restricted to the metal ion d shell. The method reproduces with considerable accuracy geometry and spin states of a wide range of Fe(II) and Co(II) complexes of various total spin and coordination polyhedra and containing both monodentate and polydentate ligands with aliphatic and aromatic nitrogen donor atoms. In this setting, a single MM parameters set is shown to be sufficient for dealing with all spin states and coordination numbers of the complexes.