to reduce the double occupancy [@Lee1] such that the electrons prefer $c _{A \uparrow} ^{\dag} c _{B \downarrow} ^{\dag} | 0 \rangle $ and $c _{B \uparrow} ^{\dag} c _{A \downarrow} ^{\dag} | 0 \rangle $ at half filling. Here $| 0 \rangle $ denotes the vacuum state, the nonnegative function $U(|{\bf r}|)$ equals zero as $|{\bf r}|$ exceeds the short interacting length $a$, and $ \psi _{ \sigma } ( {\bf r} )$ is to annihilate the electron with the spin orientation $\sigma$ at position ${\bf r}$. Such a dimer can be used to model the ionic-covalent chemical bond, which separates atom $A$ from atom $B$ in Fig. 1 (a). Assume that the bonding electrons almost concentrate in $\Omega$ such that we can consider the approximation $ \langle {\bf r} | A \rangle = \langle {\bf r} | B \rangle =0$ as $ {\bf r} \notin \Omega$, where the $A$- and $B$-site orbitals $| A \rangle$ and $|B \rangle$ denote the normalized spatial parts of $c _{A \sigma} ^{\dag} | 0 \rangle$ and $c _{ B \sigma} ^{\dag} | 0 \rangle$, respectively. In this manuscript, I assume that $| A \rangle$ and $|B \rangle$ are localized near the corresponding 