A dipole interaction model for the molecular second hyperpolarizability

A dipole interaction model (IM) for calculating the molecular second hyperpolarizability, gamma, of aliphatic and aromatic molecules has been investigated. The model has been parametrized from quantum chemical calculations of gamma 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, Phi(p), describing the width of the atomic charge distribution. The Phi(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 gamma 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 gamma 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.