  We employed first-principles density-functional theory (DFT) calculations to
characterize Dirac electrons in quasi-two-dimensional molecular conductor
$\alpha$-(BETS)$_2$I$_3$ [= $\alpha$-(BEDT-TSeF)$_2$I$_3$] at a low temperature
of 30K. We provide a tight-binding model with intermolecular transfer energies
evaluated from maximally localized Wannier functions, where the number of
relevant transfer integrals is relatively large due to the delocalized
character of Se $p$ orbitals. The spin-orbit coupling gives rise to an exotic
insulating state with an indirect band gap of about 2 meV. We analyzed the
energy spectrum with a Dirac cone close to the Fermi level to develop an
effective Hamiltonian with site-potentials, which reproduces the spectrum
obtained by the DFT band structure.
