Negative Second Hyperpolarizability of the Nitronyl Nitroxide Radical

The static second hyperpolarizability (γ) for a symmetric open-shell system including nitroxide radical groups, nitronyl nitroxide radical, has been studied using ab initio molecular-orbital methods, including various electron-correlation effects. This system is predicted to exhibit a negative γ by our classification rule of γ based on symmetric resonance structures with inversible polarization (SRIP). A remarkable electron correlation dependence has been observed in the magnitude and sign of γ, which has been found to be negative by using high-order electron correlation methods. This tendency has also been investigated in terms of the second hyperpolarizability density, which is defined as the third-derivative of the charge density with respect to the applied electric field.