2.5.1 Perturbation Theory

Deviations from an ideal lattice periodicity cause a deviation of the ideal crystal Hamiltonian $\hat{H}_{0}$. The whole system is described by a new Hamiltonian $\hat{H}$. The new Schrödinger equation cannot be solved exactly in general^2.27. However if the deviations from the periodicity are small, it is possible to find corrections to the known solution obtained for $\hat{H}_{0}$. Such corrections are obtained by the quantum mechanical perturbation theory.
S. Smirnov: