Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT) as a Simple yet Accurate Method for Diradicals and Diradicaloids

Due
to their multiconfigurational nature featuring strong electron
correlation, accurate description of diradicals and diradicaloids
is a challenge for quantum chemical methods. The recently developed
mixed-reference spin-flip (MRSF)-TDDFT method is capable of describing
the multiconfigurational electronic states of these systems while
avoiding the spin-contamination pitfalls of SF-TDDFT. Here, we apply
MRSF-TDDFT to study the adiabatic singlet–triplet
(ST) gaps in a series of well-known diradicals and diradicaloids.
On average, MRSF displays a very high prediction accuracy of the adiabatic
ST gaps with the mean absolute error (MAE) amounting to 0.14 eV. In
addition, MRSF is capable of accurately describing the effect of the
Jahn–Teller distortion occurring in the trimethylenemethane
diradical, the violation of the Hund rule in a series of the didehydrotoluene
diradicals, and the potential energy surfaces of the didehydrobenzene
(benzyne) diradicals. A convenient criterion for distinguishing diradicals
and diradicaloids is suggested on the basis of the easily obtainable
quantities. In all of these cases, which are difficult for the conventional
methods of density functional theory (DFT), MRSF shows results consistent
with the experiment and the high-level ab initio computations.
Hence, the present study documents the reliability and accuracy of
MRSF and lays out the guidelines for its application to strongly correlated
molecular systems.