The nematic coupling constant α, together with the chain stiffness κ, governs chain alignment and the isotropic-to-nematic (IN) transition temperature TIN for semiflexible polymers. We combine self-consistent field theory (SCFT) with atomistic molecular dynamics (MD) simulations of semiflexible chains under external tension in the isotropic phase to determine the nematic coupling constant α. Using α, we obtain the variational free energy of a multichain system, from which the IN transition temperature TIN can be calculated. We apply our method to obtain α and TIN of a commonly studied semiflexible conjugated polymer, poly(3-hexylthiophene) (P3HT). We predict TIN to be above the crystal melting temperature Tm for P3HT and to follow TIN(S) = 535K(1 - 1.64/S), in which S is the number of monomers.