Structural diradical character

TL;DR

This paper introduces a new measure called structural diradical character to improve the prediction of electronic diradical properties in molecules, emphasizing the importance of accurate bond-length modeling.

Contribution

It proposes the structural diradical character measure and evaluates the performance of various exchange-correlation functionals for predicting bond lengths and diradical character.

Findings

TPSSh functional performs best for organometallic systems.

B3LYP functional performs best for organic molecules.

None of the tested functionals reliably predict all bond length variations.

Abstract

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. Since diradical and closed-shell electronic structures differ in their distribution of single, double, triple and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones. Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation.