We report a diffractometric determination of the thermal dependence of
the triclinic lattice parameters of (TMTSF)(2)ClO4. Low temperature m
easurements show that c and gamma differ significantly in the quenched
(Q) and relaxed (R) states. Tight binding calculations of the band st
ructure of the organic sublattice, neglecting the ClO4- potential, sho
w that the variations of gamma between the Q and R states do not affec
t appreciably the shape of the Fermi surface (FS). We thus propose tha
t the nesting properties of the FS are destroyed below the anion order
ing transition by the b/2 component of the ClO4- Coulomb potential.