The nematic coupling constant alpha, together with the chain stiffness kappa, governs chain alignment and the isotropic-to-nematic (IN) transition temperature T-IN 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 alpha. Using alpha, we obtain the variational free energy of a multichain system, from which the IN transition temperature T-IN can be calculated. We apply our method to obtain alpha and T-IN of a commonly studied semiflexible conjugated polymer, poly(3-hexylthiophene) (P3HT). We predict T-IN to be above the crystal melting temperature T-m for P3HT and to follow T-IN(S) = 535K(1 - 1.64/S), in Which S is the number of monomers.