Phonon soft modes indicate that the structure is not stable and basically we can find lower energy structure (modulated structure) by moving atoms along the eigen vector(s) associated to the soft modes. However, there are so many choices of unitcell based on that we use to move atom to get new structures. As a results, we will have different modulated structures. Is there any way to know which unit cell will give us the lowest energy structure without doing so many trials? Some papers mention the choice of unitcell using qpoint position of the soft mode, i.e 4x4x1 unitcell if we we imaginary frequency at q(1/4,/1/4,1) but I do not know whether there is any underlying formalism behind it.