Couling, Vincent William.Jacob, Roshna Mariam.2025-12-032025-12-0320252025https://hdl.handle.net/10413/24194Doctoral degree. University of KwaZulu-Natal, Pietermaritzburg.The molecular theory of the second electric-field-induced second-harmonic generation (ESHG) virial coefficient Bγ describing the effects of interacting pairs of molecules on the molecular hyperpolarizability is reviewed, and then extended to include contributions arising from the permanent electric quadrupole moments and the dipole-dipole-quadrupole hyperpolarizabilities of the molecules. The classical long-range dipole–induced-dipole model is used to describe the interactions between pairs of molecules. The molecular (hyper)polarizability tensor components required in the calculations have been obtained by ab initio computation using DALTON. This investigation has been limited to non-dipolar species, where the permanent electric quadrupole moment is the leading multipole moment. The expressions obtained for contributions to Bγ are evaluated numerically for the H2, N2, CO2 and C2H4 molecules. For CO2 and C2H4, the quadrupole-induced and dipole-dipole-quadrupole hyperpolarizability contributions become significant, and the calculated Bγ virial coefficients indicate that the pair-interaction contributions arising for CO2 and for C2H4 are large enough to be comfortably measurable by Shelton’s ESHG apparatus and electrode arrays.enCC0 1.0 Universalhttp://creativecommons.org/publicdomain/zero/1.0/Molecular hyperpolarizability.Nonlinear optics.Ssecond virial coefficients.Thesis