A comparative study on seniority-based MO and VB calculations of the singlet and triplet energy gaps of open-shell molecules

Abstract

The recently proposed truncated configuration interaction (CI) approach based on the seniority number (Ω), which is defined as the number of singly occupied orbitals in a Slater determinant, is applied to study the adiabatic singlet-triplet excitation energies of some open-shell non-Kekulé diradical molecules, including: trimethylenemethane (TMM), tetramethyleneethane (TME) and meta-quinodimethane (m-QDM). Inasmuch as the singlet state exhibits strongly correlated electronic structure but the triplet state does not, adequate evaluation of the excitation energies requires balanced treatment of both states. It is found that within the active space comprised with the π-electrons/orbitals, the seniority-based VB approach is able to describe both the lowest singlet and triplet states in balance, but the MO analogue shows inharmonious convergence behavior toward the CASSCF limit. As a result, the seniority-based VB approach, superior to the MO analogue, can produce good singlet-triplet energy gap even at lower levels.