Bond charge model, nonlinear optical

One result of studying nonlinear optical phenomena is, for instance, the determination of this susceptibility tensor, which supplies information about the anharmonicity of the potential between atoms in a crystal lattice. A simple electrodynamic model which relates the anharmonic motion of the bond charge to the higher-order nonlinear susceptibilities has been proposed by Levine The application of his theory to calculations of the nonlinearities in a-quarz yields excellent agreement with experimental data. 

The same year, Levine [182] proposed a model that related the anharmonic motion of the bond charge, located approximately halfway between two neighboring atoms, to the second- and third-order nonlinear optical susceptibilities of... 

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. 

Chen, G., Lu. I).. Goddard, III. W.A. Valence-bond charge-transfer solvation model for nonlinear optical properties of organic molecules in polar solvents. J. Chem. Phys. 101, 5860-5864 (1994)... 

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