We demonstrated that our VFCI is able to oer a unied analysis for the spectra of a Si 2e-DQD that encompasses all three cases considered. The rst two cases concerned the full spectra, including the most important x (nm) x (nm) Energy (meV) Haux. Frames (b) and (c) display the corresponding potential connements, m * ω 2 xk x ′2 k /2 + h k , at two values of the detuning (indicated by the two upwards-pointing arrows), i.e, at ε = 0 and ε = 2.0 meV, respectively; here h1 = 0 and h2 = ε. The associated single-particle wave functions X µ(j) (x) for j = 1, . . . , 3 [see Eq. (A2)] are also displayed at these two points; see panels (d-f ) for ε = 0 and panels (g-i) for ε = 2.0 meV. We mention that the smooth-neck Hamiltonian term, V neck (x), is not included in Haux. The parameters entering in H II A 1).