Difference between microscopic and effective overlaps in the copper-oxide planes of high-Tc superconductors

We investigate parametrizations required to reproduce observed Fermi surfaces of overdoped and slightly underdoped LSCO in two different models. One is the standard three-band saddle-point slave boson (SB). The other is a model of CuO4 `molecules' randomly tiled in the plane, emphasizing the distinction between the local overlap, always the full (bare) one, and the effective overlap, connected with the reduced bandwidth due to the random tiling (RT). We conclude this distinction is physically significant for underdoped, but not for overdoped LSCO. This is consistent with the observed open Fermi surfaces being due to correlation effects.