In the early days of quantum-chemical method development for electronic structure, drastically simplified methods for large systems termed "semiempirical" like MNDO, AM1 or PM6 214 were derived as approximations to Hartree-Fock theory. Usually, minimal atom-centred atomicorbital (AO) basis sets and severe integral (multipole) approximations were applied, enabling a reasonably accurate, extremely fast treatment of mostly organic molecules. Because of the applied ZDO approximation and their Hartree-Fock origin, these methods are not robust for more complicated electronic systems like, for example, the important class of organometallic catalysts.