The TNT package uses gradient descent methods to optimize the parameters of the molecular model. For methods of this type the structure of a cycle of refinement is the same regardless of the particular method used (Agarwal, R.C., Acta Cryst. (1978). A34, 791-809). Think of each parameter of the model as a component of a very long vector. First a new vector is chosen to add to the original vector, this is the shift vector. Only a fraction of this vector, however, is to be added. The second part of each cycle is to determine how much.
If the model vector is called
and the shift vector 
, the new model vector 
is
where 
is the fraction of the shift to be applied.
The two steps of a cycle of
refinement are first to calculate and then to determine .
The different
minimization methods available in TNT alter how is calculated.
The calculation of is always done the same way.