  We present a modification of the generalized Flory dimer theory to
investigate the nematic (N) to isotropic (I) phase transition in chain fluids.
We focus on rigid linear fused hard-sphere (LFHS) chain molecules in this
study. A generalized density functional theory is developed, which involves an
angular weighting of the dimer reference fluid as suggested by decoupling
theory, to accommodate nematic ordering in the system. A key ingredient of this
theory is the calculation of the exact excluded volume for a pair of molecules
in an arbitrary relative orientation, which extends the recent work by
Williamson and Jackson for linear tangent hard-sphere chain molecules to the
case of linear fused hard-sphere chains with arbitrary intramolecular
bondlength. The present results for the N-I transition are compared with
previous theories and with computer simulations. In comparison with previous
studies, the results show much better agreement with simulations for both the
coexistence densities and the nematic order parameter at the transition.
