A study of impurity incorporation into host crystal surfaces, using molecular modeling techniques, is presented for c-caprolactam, illustrating a rapid method for predicting the effects of additives on the purity and shape of particles formed through crystallization. Optimum positions, in terms of calculated lattice energy, were located for the impurity molecules within the host crystal lattice. Differential binding energies and modified attachment energies were calculated using the program HABIT95 (Clydesdale, G.; Roberts, K. J; Docherty, R. Quantum Chemistry Program Exchange 1996, 16, 1) for each impurity molecule for all the important growth forms. From the differential binding energies, values were calculated for the equilibrium segregation coefficients. The impact of the impurities cyclohexane, cyclohexanol, cyclohexanone, and the imino-tautomeric form of epsilon-caprolactarn (or caprolactim) was considered in the study. Excellent agreement was found between calculated equilibrium segregation coefficients for the impurity cyclohexanone in the {110} and {11 (1) over bar} forms of c-caprolactam and reported experimental values for crystals grown from the melt in the presence of a wide range of cyclohexanone impurity concentrations, 0.1-30 mol %, for supersaturations of the melt, 2 x 10(-3) to 6 x 10(-3) (van den Berg, E. P. G.; Bogels, G.; Arkenbout, G. J. J. Cryst. Growth 1998, 191, 169-177). It was calculated that the incorporation of cyclohexanol into c-caprolactam would be most significant for the {101} form and that the extent of partitioning of cyclohexane into the most important growth forms is 10-100 times smaller than those in the cases of cyclohexanol and cyclohexanone. The effect of impurity incorporation on the habit of c-caprolactam crystals was calculated from the modified attachment energies.