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, 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 , indeed, it is possible to compile VMD \ without it. However, it is useful for some occasions, like making movies or scripts.
VMD uses Tcl version 7.4 and the new foreach command from 7.5 (because of its general usefulness). Since Tcl is extensible, many extension packages have been written to add improves current features and add new ones. We have included two of these, TclX (version 7.4), which adds many useful command for script writing, and Tcl-DP (version 3.2), which adds an interface to Unix socket commands. This latter extension is used by the External interfaces.
We refer you to http://www.sunlabs.com/research/tcl/ for more information about Tcl, and http://www.sco.com/Technology/tcl/Tcl.html for references to documentation on Tcl and TclX. For the documentation on Tcl-DP you'll have to download the source from ftp://mm-ftp.cs.berkeley.edu/pub/multimedia/Tcl-DP/tcl-dp3.2.tar.Z . You might also look at the examples from ???