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Tcl (short for Tool Command Language, developed by John Ousterhout) is an embeddable and extensible scripting language. In other words, Tcl sits inside VMD as a language interpreter where it can execute its standard language commands or the various VMD specific extensions. There are several reasons for using Tcl rather than writing our own language, the most important being that it is easy to use, it was easy to modify our code to use it, it has few bugs, and documentation is available at many bookstores. Many other packages use it, including Quanta. It is not necessary that you know Tcl to use VMD. However, it is useful for some occasions, like making movies or scripts.

VMD uses Tcl and Tk version 8.0.4. Since Tcl is extensible, many extension packages have been written to improves current features and add new ones. We refer you to for more information about Tcl.