Structural diradical character.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Materials science

Background:

• Accurate prediction of molecular properties like nonlinear optical and magnetic behavior relies on understanding electronic structures, particularly singlet diradical character.
• Diradical and closed-shell systems exhibit distinct bonding patterns, necessitating precise bond-length descriptions in theoretical models.

Purpose of the Study:

• To introduce and validate 'structural diradical character' as a metric for evaluating first-principles calculations.
• To identify optimal exchange-correlation functionals for predicting bond length patterns and electronic diradical character in molecules with potential photoswitchable nonlinear optical activity.

Main Methods:

• First-principles calculations using four popular approximate exchange-correlation functionals (BP86, TPSS, B3LYP, TPSSh) with varying exact-exchange admixtures.
• Evaluation of functionals based on their ability to reproduce experimental bond-length patterns and electronic diradical character.
• Application of the D3 dispersion correction for organic systems.

Main Results:

• Hybrid functionals TPSSh and B3LYP demonstrated superior performance in capturing diradical bond length patterns.
• TPSSh excelled for organometallic systems, while B3LYP performed better for organic systems (with D3 correction).
• No tested functional accurately described relative bond lengths across all studied molecules, limiting their universal applicability for diradical prediction.

Conclusions:

• Structural diradical character provides a valuable measure for assessing computational methods in diradical studies.
• While TPSSh and B3LYP show promise, current functionals have limitations in accurately predicting relative bond lengths for diverse diradical systems.
• Further development of exchange-correlation functionals is needed for reliable predictive studies of molecules with diradical character.