Hi Charlie, You should be able to use sys.exit() in a *.chimerax script to quit Chimera. The sys.exit() call raises SystemExit. Unfortunately the code that runs the *.chimerax file is catching the SystemExit. This is a bug in Chimera 1951 that we will fix. For now you can use "import os" and "os._exit(0)". The chimera_send script will start a new Chimera if it does not find one already running that is accepting connections. By default Chimera does not accept connections. The easiest way to change this is to start Chimera by hand and use Favorites/Preferences/Web Data to turn on "Accept Web Data". Then press the Preferences Save button to save this setting in the user's Chimera preferences file (~/.chimera/preferences). (Note you'll need to do this as the user that Chimera runs with from the web server.) You can also change the "Confirm open of commands or code" preference to "never". This will prevent the Chimera warning dialog asking whether you really want to execute the *.chimerax file. You can also save this setting. Making these settings may pose a serious security risk. There are mechanisms to allow only the same user that is running Chimera to run *.chimerax files but I do not know how strong the security is. Dan Greenblatt (dan@cgl.ucsf.edu) wrote the code and would know. As it works now, if you wish to send commands to an already running Chimera, you cannot specify which instance of Chimera. So it is not possible to communicate with multiple instances of Chimera as you might want to support multiple web sessions. Dan Greenblatt may have ideas for addressing this. Tom