Long-range corrected density functional theory study on static second hyperpolarizabilities of singlet diradical...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Open-shell singlet diradical systems are crucial for nonlinear optical (NLO) properties.
• Accurate calculation of their second hyperpolarizability (gamma) is computationally demanding.
• Existing methods often struggle with the complex electronic structure of diradicals.

Purpose of the Study:

• To evaluate the performance of the long-range corrected (LC) spin-unrestricted Becke-Lee-Yang-Parr (LC-UBLYP) method.
• To assess its accuracy in calculating the second hyperpolarizability (gamma) of open-shell singlet diradicals.
• To compare LC-UBLYP results with high-level computational methods across various diradical systems.

Main Methods:

• Application of the spin-unrestricted density functional theory (DFT) with the LC-UBLYP method.
• Calculations performed on model systems (H2 dissociation, 1,3-dipoles, p-quinodimethane) and real systems (BI2Y, phenalenyl-linked diradicals).
• Comparison of LC-UBLYP results with reference methods like Full Configuration Interaction (FCI), Coupled Cluster (UCCSD, UCCSD(T)), and other DFT functionals (UBHandHLYP, UB3LYP).

Main Results:

• LC-UBLYP qualitatively reproduces the evolution of gamma with diradical character in H2 dissociation.
• Semiquantitative agreement with UCCSD/UCCSD(T) for small 1,3-dipoles and good reproduction for p-quinodimethane and BI2Y systems.
• LC-UBLYP results closely match UBHandHLYP for phenalenyl-linked diradicals, outperforming UB3LYP in some cases.

Conclusions:

• The LC-UBLYP method offers an efficient and reliable approach for determining the NLO properties of open-shell singlet diradicals.
• It demonstrates good performance across a range of diradical complexities and electronic structures.
• Findings highlight the need for improved exchange-correlation functionals for challenging open-shell systems.