Quasi-relativistic formulation of the Magnetic circular dichroism (MCD) Faraday terms are presented using the generalized unrestricted Hartree-Fock (GUHF)/single excitation configuration interaction (SECI) method combined with the finite perturbation method and applied to the MCD of the three n-sigma states ((3)Q(1), (3)Q(1), (1)Q(1)) of CH3I. The Faraday B term for the (1)Q(1) state was 0.1976(Debye)(2) (Bohr magneton)/(10(3) cm(-1)) in the non-relativistic theory. but was dramatically improved by the relativistic effect and became 0.0184 in agreement with the experimental values, 0.014 and 0.0257. This change was mainly due to the one-electron spin-orbit (SOI) term rather than the spin-free relativistic (SFR) and the two-electron spin-orbit (SO2) terms. (C) 2002 Elsevier Science B.V. All rights reserved.