A Dipole Interaction Model for the Molecular Second Hyperpolarizability

A dipole interaction model (IM) for calculating the molecular second hyperpolarizability, γ, of aliphatic and aromatic molecules has been investigated. The model has been parametrized from quantum chemical calculations of γ at the self-consistent field (SCF) level of theory for 72 molecules. The model consists of three parameters for each element p: an atomic polarizability, an atomic second hyperpolarizability, and an atomic parameter, Φ p, describing the width of the atomic charge distribution. The Φ p parameters are used for modeling the damping of the interatomic interactions. Parameters for elements H, C, N, O, F, and Cl were determined, and typical differences between the molecular γ derived from quantum chemical calculations and from the IM are below 30% and on average around 10%. As a preliminary test, the dipole interaction model was applied to the following molecular systems not included in the training set: the urea molecule, linear chains of urea molecules, and C 60 . For these molecules deviations of the IM result for the molecular γ from the corresponding SCF value were at most around 30% for the individual components, which in all cases is a better performance than obtained with semiempirical methods.