VB-CT model

The two-form model has its roots in the valence-bond charge-transfer (VB-CT) model derived by Mulliken [84] and used with minor modifications by Warshel et al. for studying reactions in solutions [114]. Goddard et al. applied this VB-CT model to study the nonlinear optical properties of tire charge-transfer systems. [27, 59]. The analysis of the relationship between electronic and vibrational components of the hyperpolarizabilities within the two-state valence-bond approach was presented by Bishop et al. [17]. Despite the limitations of the VB-CT model, it is very simple and gives some insight into mutual relationships between nonlinear optical responses through the various orders. 

The alternation of the single and double bonds in the polyene chain is inverted in the VB and CT forms, see Fig. 3.21, so that the model molecule can also be described in terms of the bond length alternation (BLA). The BLA is defined as ... 

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