The program Shift contains all the code which implements the function minimizer of TNT. Shift does not know anything about diffraction, stereochemistry, or noncrystallographic symmetry. It is quite flexible about what function it is minimizing. The form and definition of the parameters to be varied are built into Shift and these cannot be changed. TNT will allow you to change the data your model is restrained to but will not allow you to change the nature of the parameters themselves. (Someday this may change.)