OPT, OPT-X

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