Hi to all community of Physic's help from Florence, looking at born-von karman BC i'm a bit confused. I put this condition when i assume periodicity of wave function where the period is the spatial dimension of my system. I found that BC first in solid state physic, then i've noticed that became the natural BC for "all" quantum system. I dont understand what i physically say. It's not obvious (to me) that a wavefunction must have some symmetriy (although it's a bit intuitive). My "state of matter lectures" said: "We can think at nullify wavefunction out of crystal as possible BC. This condition brings stationary solutions while we need progressive waves for describing phenomena like energy or charge transport". ...and so we use the born von-karman. Its really sounds obscure to me :grumpy: Ll.