calculation for \[Au(tmdt)$_2$\] indicates a stable AF pattern where spins align oppositely within each molecule. To understand this unusual situation, it was speculated that a ligand $p\pi$-$p\pi$ transfer integral is larger between adjacent molecules than that within a molecule, which is supported by the results of an analysis based on the fMO approach [@Seo_2008JPSJ]. These features are consistent with the molecular design [@Kobayashi_2001JMC] mentioned in § \[sec1\] and imply that the fMO approach gives an insightful picture of the electronic states of SCMC. Such discussions have recently been elaborated within quantum chemistry calculations [@Bonnet_2010JCP; @Tsuchiizu_2011JPSJ; @Tsuchiizu_2012JCP], for (TTM-TTP)I$_3$ and \[Au(tmdt)$_2$\]. It is shown that the two approaches, the use of the MO and fMO pictures, can be transformed from one to the other. We note that the fMO approach here and the so-called fragment molecular orbital method applied to huge molecules [@Kitaura_1999CPL; @Tsuneyuki_2009CPL] share common concepts.

In the following, all first-principles calculations, including those for MO of isolated molecules, are performed using the computational code QMAS (Quantum MAterials Simulator) [@QMAS] based on the projector augmented-wave method [@PAW] with the generalized gradient approximation [@GGA]. See refs.  and for details.

![(Color online) Molecular orbitals for $M$(tmdt)$_2$ ($M$ = Ni, Au, and Cu) 