Investigating Disjoint Non-Kekulé Diradicals with Quantum Monte Carlo: The Tetramethyleneethane Molecule through the Jastrow Antisymmetrized Geminal Power Wave Function

Disjoint non-Kekulé molecules are diradicals that present two independent radical centers and can violate Hund’s rule, according to which the ground state should have triplet spin symmetry. The prototype of this class of systems is the tetramethyleneethane (TME) molecule for which indeed ion photoelectron spectroscopy (IPS) experiments revealed the singlet 1A state to be more stable than the triplet 3Bu. In this work we investigate the potential energy curves of the two spin states of TME and of the two anionic states of TME–(2A and 2B1) as a function of the torsion of the central dihedral angle, with quantum Monte Carlo methods and a Jastrow Antisymmetrized Geminal Power wave function. Through ab initiogeometrical optimizations we study the possible structural interconversions between the states, finding results which are in full agreement with the IPS experimental data.