The command ac enables the use of accelerators, simple keyboard shortcuts for many operations in Chimera. This command is understood only after the Accelerators extension (Accelerator List or Accelerators On) has been started at some point in the session.
When accelerators are on and the Command Line is open, one must click in the graphics window to indicate that keystrokes should be interpreted as accelerators and click in the command-line area to indicate that keystrokes should be interpreted as commands.
Accelerators can be disabled with the accelerator af or by choosing Tools... General Controls... Accelerators Off from the menu.