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A package of programs has been developed for efficient restrained least-squares refinement of macromolecular crystal structures. The package has been designed to be as flexible and general purpose as possible. The process of refinement is divided into basic units and an independent computer program handles each task. Each functional unit communicates with other programs in the package by way of files of well defined format. To modify or replace any program, the user need only understand the function of that particular element. Stereochemical restraints are defined in a general way that can be applied to proteins, nucleic acids, prosthetic groups, solvent atoms and so on. Guide values for bond lengths and bond angles are specified in a straightforward direct manner. Designated groups of atoms can be held constant or constrained to behave as a rigid body during refinement. In order to make the package as efficient as possible, the fast Fourier transform algorithm is used for all the crystallographic transformations. To highlight potential errors in the refined structure the user can list those atoms that have the worst bond lengths and angles, or have the largest positional, temperature-factor or occupancy gradients. It is also possible to check that protein and solvent atoms do not sterically clash with symmetry-related neighbors. Applications of the program package to a bacteriochlorophyll-containing protein, thermolysin-inhibitor complexes and mutants of bacteriophage T4 lysozyme are described.
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