In order to avoid the difficulties encountered in the crystallization computer simulation of macromolecular crystals, we introduce the so-called "growth cell" that is, the simple geometric volume occupied by a structural growth unit (i.e., monomer, dimer, etc.) in the crystal. The number and orientation of bonds that are satisfied by the surrounding ones are accounted in the surfaces of these growth cells. This construction allows us to compare the attachment probabilities of growth units in different crystallographic orientations on macromolecular crystals surfaces. The application of our method to the recent experimental observation of the formation of steps in the (010) direction on the (101) concanavalin A crystal surface is presented.