Excited states and resonance interactions

Eo being the energy limit on the groumf-state surface. 

To exploit the formalism used so far, the wavefunction will be written 

The leading term in E is an electrostatic interaction similar to (14.3.10), except that A is now in state a instead of a the next two sums in (14.6.3) give rise to polarization energies for A (in state a ), in the presence of B (in state b), and for B (in state b) in the presence of A (in state a ). The final term introduces interesting new possibilities the energy denominators in the second-order sum are now, instead of those that appear in (14.4.1), 

Such an effect is well known in the theory of molecular crystals (see e.g. Davydov, 1962), where it is usually referred to as Davydov splitting . 

The matrix elements in (14.6.6) are evaluated as in earlier sections, and give immediately a first-order interaction energy 

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