A methodology is outlined that open a feasible way to the study of different impurity-crystal surface interactions. This study is made with the aid of the so-called "growth cell" and using semiempirical quantum chemical calculations. Here, we attempt to reproduce the observed experimental behaviour in order to clarify at molecular level the role of raffinose in the growth of sucrose crystals. Our results proved the directing role of the number of hydrogen bonds in the stabilization of the raffinose-sucrose crystal interaction.