A semiempirical method based on geminal functions

An attempt has been made to develop a semiempirical method which considers only the n- and π-electrons, with the eigenfunctions expressed as an antisymmetrized product of two-electron functions or geminals. These geminals are expressed as a linear combination of products of Hückel-type MO's and the matrix elements are evaluated assuming the strong orthogonality condition to hold among the geminals, with an average effective Hamiltonian where the interaction between paired electrons is explicitly included. A first application of the method to N₂, HCN, C₂H₂, and C₂H^? was carried out without the introduction of parameters adjustable to best fit. A parametric approximation was then used for studying the electronic structure of diazabarrelene, an unknown molecule containing three pairs of π-electrons and two pairs of n-electrons. It is concluded that the explicit introduction of (π,π) and (n-π)-electron interaction describes the ground state more realistically than the simplest Hückel method. The separation of the energy levels was also affected, but the calculated transitions compared rather poorly with the spectroscopic observations. Diazabarrelene is expected to be not less stable than barrelene and attempts to its synthesis should be considered worthy of being made.