depends on the occupancy on the two related sites. Using typical parameter values for the $d$-$p$ model $t$ is estimated at 0.51 eV for hole transfer. $U$ is estimated at 3.4 eV so that $U/t = 6.7$. With this relatively small $U$ the authors derive an appropriate value of exchange interaction constant $J$ for the $t$-$J$ model, appearing later, thanks to the ferromagnetic interaction term and the Coulomb interaction between nearest neighbor sites in the effective one-band Hubbard model. These values of $t$ and $U$ are common for the cuprate high-$T_c$ superconductors. The $t'$ value was estimated at around $-0.06$ eV, i.e., $-0.12$ in units of $t$, for hole-type superconductors. They observed that its weak dependence on each cuprate is correlated with the values of $T_c$ among the cuprates. Since the additional spin-dependent ferromagnetic term and nearest neighbor Coulomb interactions are relatively small, the 2D Hubbard model without these additional interaction but with the above-mentioned parameter values should give a right magnitude of the SC energy gain if it actually gives a finite value.

For $U = 8$ and $t' = 0$ the maximum SC energy gain per site is approximately given by the value at $\rho=0.84$ in the neighborhood 