In the harmonic lattice approximation the atomic deviations are assumed to beso small that the potential energy is well described by the second order term in(8.1). This is generally a good approximation, at least at relatively low temper-atures. Later on in this chapter examples of situations will be given in whichthe harmonic approximation fails, such as the high temperature bcc phase ofTi, Zr and Hf. Furthermore, in order to make the notation more transparent,the notation of a monoatomic lattice will be adapted without any loss of gen-erality. The harmonic Hamiltonian in the case of a monoatomic lattice is given by
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