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 version 7.6 and Tk version 4.2.
Since Tcl is extensible, many extension packages
have been written to improves current features and add new ones. We
have included two of these, TclX (version 7.6), which adds
many useful command for script writing, and Tcl-DP
(version 4.0), which adds an interface to Unix socket commands. This
latter extension is used by the
External interfaces.
The TkX version 4.2.0 is also included.
We refer you to
http://www.scriptics.com/
for more information about Tcl, and
http://www.sco.com/Technology/tcl/Tcl.html
for references to documentation on Tcl/Tk and TclX. For the
documentation on Tcl-DP see the Tcl-DP home page at
http://www.cs.cornell.edu/Info/Projects/zeno/Projects/Tcl-DP.html.