$| 2 \rangle \otimes | - \sigma \rangle =  | L \rangle \otimes | - \sigma \rangle $. The two unoccupied levels $| \bar{1} \rangle \otimes | \sigma \rangle$ and $| \bar{2} \rangle \otimes | - \sigma \rangle$ in Fig. 2(a) serve as $ | B \rangle \otimes | \sigma \rangle$ and $| \bar{L} \rangle \otimes | - \sigma \rangle$ in such a one-bond system, where $| \bar{L} \rangle = \alpha _{c} ^{*} | A \rangle - \alpha _{i} ^{*} | B \rangle \bot | L \rangle$ is a ket in $\mathbb{C} ^{2} _{\Omega}$. The uncorrelated energy $\langle \Phi _{b} | H _{b} | \Phi _{b}  \rangle = \langle \Phi ^{\prime} _{b} | H _{b} | \Phi _{b} ^{\prime}  \rangle$ equals $$\begin{aligned}
 tr  (\rho ^{(1)} + \rho ^{(2)}) H_{sb} + \int _{ {\bf r} ^{ \prime },{\bf r} ^{ \prime \prime} \in \Omega} d ^{3} r ^{\prime} d ^{3} r ^{ \prime \prime } U ( | {\bf r} ^{\prime} - {\bf r} ^{\prime \prime} | ) \langle {\bf r} ^{\prime} | \rho ^{(1)} | {\bf r} ^{\prime} \rangle \langle {\bf r} ^{\prime \prime} | \rho ^{(2)} | {\bf r} ^{\prime \prime} \rangle,\end{aligned}$$ where $H_{sb} = t _{AB}  | A 