  Recent Scanning Tunneling Spectra(STS) measurement on underdoped cuprate
discovers the increase of the maximum superconducting transition temperature
$T_c$ when the size of charge transfer gap (CTG) is reduced. Applying pressure
is another well known method to increase maximum $T_c$. However, these pressure
experiments also found another puzzle that $T_c$ is enhanced in underdoped and
optimal doped samples but suppressed in overdoped. Here we present a possible
mechanism based on the charge fluctuation to explain both these two effects
simultaneously. Starting from 3-band Hubbard model, we retrieve the charge
fluctuation(CF) between oxygen 2$p^6$ band and copper 3$d^{10}$ band which is
ignored in the $t-J$ model. This model is studied via variational Monte Carlo
method(VMC).
