Non-Heitler-London term

This term breaks down tire excitonic approximation mixing up states whose exciton number differs by two units. This so called non-Heitler-London term has negligible effects for systems with J 2wo [47]. So the excitonic approximation is expected to work well for clusters of molecules with large excitation energies and not too large transition dipole moments, i.e. for hardly polarizable molecules. 

The tlrlrd portion of the total Hamiltonian, in (14), collects all terms beyond the excitonic approximation, being responsible for the mixing of states with different number of excitons. The first term in this ultraexcltonic Hamiltonian corresponds to the non-Heitler London term in Eq. (2), as originating from the interaction between transition dipole moments. The second term describes the interaction between transition and mesomeric dipole moments and corresponds to the term in Eq. (4) wiflr = V (l -2p)yp(l -p) (cf. Eqs. (8)-(7)). 

Terenziani et al have employed a model Hamiltonian including Heitler-London and non-Heitler-London terms to interpret experimental results on the effects of the environment on the second-order NLO response. In particular, they analyzed i) the self-orientation of the chromophores in... 

---