The free rotor model is the pseudo-ellipse model with the torsion angle restriction absent. Its parameters are

where *P*_{i} are the average domain position translations and rotations, *E*_{i} are the Euler angles defining the motional eigenframe, *p*_{i} are the coordinates of the pivot point, and *θ*_{x} and *θ*_{y} are the maximum cone opening half-angles.

The relax user manual (PDF), created 2016-10-28.