Turn off all optimization flags for atoms of type X.  This keyword is useful for partial geometry optimizations.  If "-X'" is missing, turn off all optimization flags. If OPT-X  is present, optimization flags can be turned on after NOOPT has been run, so to optimize the positions of hydrogen atoms while freezing the positions of all other atoms, use "NOOPT OPT-H".

If  OPT("label"=n.nn)is present, then this will be run first, then NOOPT will turn off all optimization flags for atoms containing the label "label".

In IRC, DRC, and LOCATE-TS jobs, NOOPT has a different meaning.  In these jobs the default is that all atoms, including those whose optimization flags indicate that they should be frozen, are allowed to move.  To prevent this from happening add NOOPT.  Now instead of NOOPT turning off optimization flags, it prevents optimization flags that are off from being turned on.