To the moderator: I'm not sure if this should go here or in the Computational Physics forum. Please shift it there if you think thats the appropriate place for it. Hi everyone Merry Christmas! I'm writing a computer program in C, to explicitly compute the band structures for a 1D crystal modelled as an infinite array of rectangular potential barriers of width 2a and distance between nearest walls of two successive barriers 2b, i.e. the Kronig Penney Model. My program correctly plots the energy versus (normalized) wavenumber graph in the extended wavenumber scheme, but when I try to superimpose the free particle parabola on this graph (i.e. the parabolic graph of energy versus wavenumber for a free particle), it is not found to intersect the band curves at points of the form [itex]n\pi[/itex] where [itex]n[/itex] is an integer. (I call gnuplot from within my C program to plot this curve.) The plot generated by the program is as shown in the attachment. I think this is because of some numerical approximations and propagation errors, but I haven't been able to fix it. As you can see from the plot, the two curves do intersect at [itex]\pi[/itex] (or very nearly so) but thats just it. I would appreciate any ideas or suggestions. I can post my algorithm/code here if you wish. Thanks in advance, Cheers Vivek.