The simplest molecular orbital theory of $\ce{\uppi\!\mbox{-}conjugated}$ molecular systems. It uses the following approximations: $\ce{\uppi\!\mbox{-}electron}$ approximation; LCAO representation of the $\ce{\uppi\!\mbox{-}molecular orbitals}$; neglect of electron-electron and nuclear-nuclear repulsions (in fact, the assumption that these cancel). The diagonal elements of the effective Hamiltonian, coulombic integrals, and the off-diagonal elements, resonance integrals, (accounted for only directly bonded atoms) are chosen as empirical parameters, all overlap integrals being neglected.