Dispersion relations have the tendency to confuse me. In general, I know what dispersion is, but trying to apply it to crystals, I just "can't see the forest among all those trees". :uhh: In phonon dispersion, acoustical and optical phonons have quite a different dispersion behaviour. Why is that? I do know the difference between acoustical and optical phonons, but I don't see the physical meaning. Electron dispersion "creates" the energy bands. But again, I don't really have a concept of the physical meaning. Can anyone depict this, please? And also: what exactly do you need dispersion relations for? What information do you get from them?