Anharmonic contribution to the static vibrational second hyperpolarizability of linear quadrupolar molecules within the framework VB-2CT and AEEP

Anharmonic (γvanhar) and harmonic (γvhar) contributions to the static vibrational second hyperpolarizability (γv) of symmetrically substituted quadrupolar π-conjugated molecules are investigated using a three-state (VB-2CT) model and the analytical evaluation of electrical properties (AEEP) method. Calculations show that using strong donors and acceptors for given quadrupolar molecule γvanhar can be more important than its electronic counterpart (γe). Additionally, γvanhar is comparable in magnitude to γvhar.