GDIS is a freely available chemical visualization program for displaying molecules, periodic structures, and crystal morphologies. A key feature of the package is the mechanism for constructing and manipulating arbitrary crystal surfaces. This enables GDIS to offer a powerful tool set for computing surface and interfacial properties and predicting crystal morphologies. Also included are modules for computing powder diffraction patterns, generating molecular surfaces, and analyzing dynamics trajectories. Further, the program may be used as a graphical interface to simplify the construction of input files for the command line codes: GULP, GAMESS(US), and SIESTA.

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Epitaxial relationships between the surfaces of inorganic and bioorganic crystals can be an important factor in crystal nucleation and growth processes in a variety of biological environments. Crystalline cholesterol monohydrate (ChM), a constituent of both gallstone and atherosclerotic plaques, is often found in association with assorted mineral phases. Using in situ atomic force microscopy (AFM) and well-characterized model bile solutions, the nucleation and epitaxial growth of ChM on calcite (104) surfaces in real-time is demonstrated. The growth rates of individual cholesterol islands formed on calcite substrates were determined at physiological temperatures. Evidence of Ostwald's ripening was also observed under these experimental conditions. The energetics of various (104) calcite/(001) ChM interfaces were calculated to determine the most stable interfacial structure. These simulations suggest that the interface is fully hydrated and that cholesterol hydroxyl groups are preferentially positioned above carbonate ions in the calcite surface. This combination of experimental and theoretical work provides a clearer picture of how preexisting mineral seeds might provide a viable growth template that can reduce the energetic barrier to cholesterol nucleation under some physiological conditions.

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