Establishing the ground state of the disjoint diradical tetramethyleneethane with quantum Monte Carlo.

The nature of the electronic ground state of the tetramethyleneethane (TME) diradical has proven to be a challenge for both experiment and theory. Through the use of quantum Monte Carlo (QMC) methods and multireference perturbation theory, we demonstrate that the lowest singlet state of TME is energetically lower than the lowest triplet state at all values of the torsional angle between the allyl subunits. Moreover, we find that the maximum in the potential energy curve for the singlet state occurs at a torsional angle near 45°, in contrast to previous calculations that placed the planar structure of the singlet state as the highest in energy. We also show that the CASPT2 method when used with a sufficiently large reference space and a sufficiently flexible basis set gives potential energy curves very close to those from the QMC calculations. Our calculations have converged the singlet-triplet gap of TME as a function of methodology and basis set. These results provide insight into the level of theory required to properly model diradicals, in particular disjoint diradicals, and provide guidelines for future studies on more complicated diradical systems.