Inaccurate name: Modes

[aglie]

Usually modes are related to the normal mode analysis of dynamic matrix of the crystal. There they decompose all atomic displacements into fourier harmonics, which are multiplied by special variables - xi - normal modes of crystal vibrations.

Normal modes in usual cense allow to define not only the direction of displacement of the atoms, but also the phase shift between those displacements. We do not have the phase shift term, thus the name is inaccurate.