Tetramethylene: A CASPT2 study

The potential energy surface (PES) of the 1,4-tetramethylene biradical has been reinvestigated at the complete-active-space self-consistent field (CASSCF), complete active space with second-order perturbation theory correction and multi-reference configuration interaction (MRCI) levels of theory. The choice of basis set, the size inconsistency of the MRCI wavefunction and the selection of the active space of the CASSCF wavefunction each have a considerable influence on the shape of the PES. In particular, the relative energy of the so-called GF and TF structures depends significantly on the level of theory employed. The results of the study suggest that the experimentally observed intermediate is more likely to be due to entropic trapping rather than the presence of a transition state.