  Using the dx2-y2+dz2 two orbital model of the high Tc cuprates obtained from
the first-principle calculation, we show that the material dependence of the
Fermi surface shape can be understood by the degree of the mixture between the
dx2-y2$ and the dz2 orbitals. We explain, through investigating the
tightbinding hopping integrals, why some cuprates have square shaped Fermi
surface, while others have more rounded ones. From this viewpoint, we explain
the experimentally observed correlation between the curvature of the Fermi
surface and Tc.
