A modified tight-binding recursion method is used to calculate the d electron contribution to force components on various segments of an unrelaxed screw dislocation in the presence of a solute atom. The model parameters correspond to an iron solute atom embedded in a molybdenum lattice. The solute atom is introduced at various sites in the core region of the dislocation. This quantum mechanical calculation of the solute-atom-dislocation interaction force is contrasted with a more conventional elastic continuum model as well as with a semi-empirical (Johnson) pair potential atomistic model. Significant differences are obtained in both the magnitude and the orientation dependence of the force, as computed with the atomistic model as opposed to the continuum model. Somewhat fewer differences were obtained for the quantum mechanical as opposed to the pair potential atomistic model.