This paper reports the effects of added amino acids on the habit modification in glycine crystals obtained via solution crystallization. A molecular modeling approach using the atom-atom potential energy method is described to rationalize the experimental observations. Crystal growth is monitored in situ using atomic force microscopy, providing insights into the interaction of impurities at the crystal-solution interfaces and consequently correlating the 'pinning' effect of impurities at the molecular scale to the macroscopically observed habit modification. The potential for development of the approach toward prediction of impurity effects on the growth morphology of molecular crystals is discussed. (c) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008