H=p^2/2m+c What's c? It's of course a shift in energy, but can be thought also as a smoother and smoother real-space local potential that becomes a constant all over the space. On the other hand, why couldn't one think about it as a constant potential in reciprocal space? It's a shift in energy so it's a constant everywhere... But how to reconcile this with the fact that the Fourier transform of a constant is a Dirac delta?