The program Coot (Crystallographic Object-Oriented Toolkit) is used to display and manipulate atomic models of macromolecules, typically of proteins or nucleic acids, using 3D computer graphics. It is primarily focused on building and validation of atomic models into three-dimensional electron density maps obtained by X-ray crystallography methods, although it has also been applied to data from electron microscopy.

Coot displays electron density maps and atomic models and allows model manipulations such as idealization, real space refinement, manual rotation/translation, rigid-body fitting, ligand search, solvation, mutations, rotamers, and Ramachandran idealization. The software is designed to be easy-to-learn for novice users, achieved by ensuring that tools for common tasks are 'discoverable' through familiar user interface elements (menus and toolbars), or by intuitive behaviour (mouse controls). Recent developments have enhanced the usability of the software for expert users, with customisable key bindings, extensions, and an extensive scripting interface.

Coot is free software, distributed under the GNU GPL. It is available from the Coot web site originally at the University of York, and now at the MRC Laboratory of Molecular Biology. Pre-compiled binaries are also available for Linux and Windows from the web page and CCP4, and for Mac OS X through Fink and CCP4. Additional support is available through the Coot wiki and an active COOT mailing list.

The primary author is Paul Emsley (MRC-LMB at Cambridge). Other contributors include Kevin Cowtan, Bernhard Lohkamp and Stuart McNicholas (University of York), William Scott (University of California at Santa Cruz), and Eugene Krissinel (Daresbury Laboratory).

Coot can be used to read files containing 3D atomic coordinate models of macromolecular structures in a number of formats, including pdb, mmcif, and Shelx files. The model may then be rotated in 3D and viewed from any viewpoint. The atomic model is represented by default using a stick-model, with vectors representing chemical bonds. The two halves of each bond are coloured according to the element of the atom at that end of the bond, allowing chemical structure and identity to be visualised in a manner familiar to most chemists.

Coot can also display electron density, which is the result of structure determination experiments such as X-ray crystallography and EM reconstruction. The density is contoured using a 3D-mesh. The contour level controlled using the mouse wheel for easy manipulation - this provides a simple way for the user to get an idea of the 3D electron density profile without the visual clutter of multiple contour levels. Electron density may be read into the program from ccp4 or cns map formats, though it is more common to calculate an electron density map directly from the X-ray diffraction data, read from an mtz, hkl, fcf or mmcif file.

Coot provides extensive features for model building and refinement (i.e. adjusting the model to better fit the electron density), and for validation (i.e. checking that the atomic model agrees with the experimentally derived electron density and makes chemical sense). The most important of these tools is the real space refinement engine, which will optimize the fit of a section of atomic model to the electron density in real time, with graphical feedback. The user may also intervene in this process, dragging the atoms into the right places if the initial model is too far away from the corresponding electron density.

Tools for general model building: