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The authors include Umklapp processes into the coupled exciton-photon (polariton) theory. Additional radiation field terms resonate at Bragg conditions but may also significantly change the character of any resonance at optical frequencies. Comprehensive treatment of the radiation field yields effective v-body intermolecular interactions which are fully retarded. The Lamb shift of the ground state is a natural consequence of the formalism. Polariton theory agrees with linear response if and only if Umklapp processes are included.
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Bullough et al. (1970) studied this question.
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