CHAIN: a crystallographic modeling program.

Publisher Summary The program CHAIN was developed to assist the crystallographer in building atomic models from an electron-density map. By using a set of simple commands, the user easily can build, display, and manipulate atomic models and electron-density maps on a computer graphics device. The user first prepares data files containing the desired coordinate and density information. CHAIN then displays the electron-density contours, superimposed with atomic coordinates, on the screen. The modifications to the coordinates can then be made interactively. A number of additional features are included with the program to regularize or refine the coordinates automatically and to display them in a variety of ways. The program also provides a number of interfaces to read in coordinates from and write out coordinates to non-CHAIN programs. An attempt has been made to allow a great deal of flexibility in the program while keeping it simple and easy to use, particularly for the novice user. Command files, display menus, function keys, and interactive dials allow for easy use without the need to remember a large number of commands. In addition, both the terminal and the interactive display have HELP commands to obtain information on any option.