The reference species used to evaluate the resonance energy (RE) are optimized so as to make them completely free of any type of resonance, strain, hyperconjugation, differences in the types of bonds and hybridization, etc. This is achieved by introducing a fictive set of energies of the reference species referred to as the "equilibrium" energies. The latter are determined by requiring the energy change of any conceivable reaction involving the reference species to be zero and employing an optimization procedure similar to the least-squares method. It is shown further that the RE defined in terms of "equilibrium" energies of the reference species have the remarkable property of being independent of the stoichiometry of the reactions used to evaluate the RE.