  From configuration interaction (CI) ab initio calculations, we derive an
effective two-orbital extended Hubbard model based on the gerade (g) and
ungerade (u) molecular orbitals (MOs) of the charge-transfer molecular
conductor (TTM-TTP)I_3 and the single-component molecular conductor
[Au(tmdt)_2]. First, by focusing on the isolated molecule, we determine the
parameters for the model Hamiltonian so as to reproduce the CI Hamiltonian
matrix. Next, we extend the analysis to two neighboring molecule pairs in the
crystal and we perform similar calculations to evaluate the inter-molecular
interactions. From the resulting tight-binding parameters, we analyze the band
structure to confirm that two bands overlap and mix in together, supporting the
multi-band feature. Furthermore, using a fragment decomposition, we derive the
effective model based on the fragment MOs and show that the staking TTM-TTP
molecules can be described by the zig-zag two-leg ladder with the
inter-molecular transfer integral being larger than the intra-fragment transfer
integral within the molecule. The inter-site interactions between the fragments
follow a Coulomb law, supporting the fragment decomposition strategy.
