  We present a first-principles band calculation for the quasi-one-dimensional
(Q1D) organic superconductor (TMTSF)$_2$ClO$_4$. An effective tight-binding
model with the TMTSF molecule to be regarded as the site is derived from a
calculation based on maximally localized Wannier orbitals. We apply a
two-particle self-consistent (TPSC) analysis by using a four-site Hubbard
model, which is composed of the tight-binding model and an on-site
(intramolecular) repulsive interaction, which serves as a variable parameter.
We assume that the pairing mechanism is mediated by the spin fluctuation, and
the sign of the superconducting gap changes between the inner and outer Fermi
surfaces, which correspond to a d-wave gap function in a simplified Q1D model.
With the parameters we adopt, the critical temperature for superconductivity
estimated by the TPSC approach is approximately 1K which is consistent with
experiment.
