Let us consider energy gap among the lowest three energy levels. For the particle in a ring model, the ratio of (E2 -E1) to (E1 -E0) is fP = 2 2 -1 = 3 according to eq. 4. For HMO method, fH(θ) satisfies eq. 5, where θ = 2π/n represents the central angle corresponding to one side of the regular n-gon inscribed in a circle. The lowest three levels are used to evaluate the precision of related models, in order to eliminate errors caused by inaccuracies in the computational values of high-energy orbitals, especially virtual orbitals.