Exciton states beyond the Heitler London approximation

3 Exciton states beyond the Heitler London approximation 

In the previous section we have obtained the crystal excitation energy in the Heitler-London approximation, i.e. neglecting the term H3 in the crystal energy operator (3.19). We now wish to determine the crystal energy without using this simplification. 

Diagonalization of the operator (3.58) can be performed by means of the unitary transformation17 

17For the diaganolization of quadratic forms of Bose operators, see Appendix A. 

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