Crystals with several molecules per unit cell

2 Crystals with several molecules per unit cell 

If a crystal contains a molecules per unit cell, the indices n and m in (3.35) are given in the form 

Diagonalization of the operator (3.35) can be carried out in two steps. First we use the canonical transformation 

Substituting (3.42) into (3.35) we obtain the following expression for the excitation energy operator 

Since we consider only one /th excited state, the index / will be omitted. Making use of this notation, we put the operator (3.44) into the form 

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With the aim of obtaining general expressions for corrections to the Heitler-London approximation in crystals with several molecules per unit cell, we put eqns (3.62) into the form... 

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