William B. Hughes, Mihai V. Popescu, Robert S. Paton (2025)
Highlighted by Jan Jensen
While this paper presents an interesting and useful benchmark and dataset of barrier heights involving organic molecules in the triplet state, I am highlighting this paper for a different reason.
While compiling the data set the authors "observed a common tendency for triplet SCF calculations to converge non-Aufbau solutions, resulting in catastrophic predictions in both thermochemistry and activation energy barriers and leading to errors as high as 26.4 kcal/mol." They go on to note that "Since such errors cannot be predicted a priori, manual inspection of spin densities for triplet-state calculations can be helpful to ensure the lowest triplet state has been converged with KS-DFT."
I remember the days when the SCF would routinely fail to converge even for simple singlet ground state molecules. So when they did converge and you got odd results, one of the first things you checked was the orbitals. But those days are long gone and I don't think it would occur to me now. I'd be much more likely to ascribe it to some deficiency in the functional.
I now wonder how many of such wrong conclusions are scattered throughout the literature, especially for molecules with "funky" electronic structure, such as transition metal complexes. Manual inspection of the MOs is not going to be a practical option for many of these studies, and SCF stability checks did not identify all problems!
However, most QM packages have several options for the MO guess and it might be a good idea to use more than one of them and check whether they all converge to the same SCF solution. It'll be just like the old days.
This work is licensed under a Creative Commons Attribution 4.0 International License.
