I'm taking solid state, and again and again we use the periodic boundary conditions, that the wavefunction should be unchanged by displacements of the length of the sample, L (assume 1D for simplicity). The argument was that the surface is so far away that it shouldn't have an effect on the properties in the interior, so we are free to use boundary conditions that are mathematically convenient. But this argument doesn't work for me, because if the surface is so far away as to be irrelevant, why should 'L' enter into things at all? Why should the properties be different deep inside a 1 cm sample from inside an otherwise identical 2 cm sample?(adsbygoogle = window.adsbygoogle || []).push({});

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# Periodic boundary conditions

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