  We describe the LDA bandstructure of YBa_2Cu_3O_7 in the 2 eV range from the
Fermi energy using orbital projections and compare with YBa_2Cu_4O_8. Then, the
high-energy and chain-related degrees of freedom are integrated out and we
arrive at two, nearest-neighbor, orthogonal, two-center, 8-band Hamiltonians,
the even and odd bands of the bi-layer. Of the 8 orbitals, Cu{x2-y2}, O2x, O3y,
and Cus have \sigma character and Cu{xz}, Cu{yz} O2z, and O3z have \pi
character. The roles of the Cu_s orbital, which has some Cu{3z2-1} character,
and the four \pi orbitals are as follows: Cu_s provides 2nd- and
3rd-nearest-neighbor (t' and t') intra-plane hopping, as well as hopping
between planes (t_{perp}). The \pi -orbitals are responsible for bifurcation of
the saddle-points for dimpled planes. The 4-\sigma-band Hamiltonian is generic
for flat CuO_2 planes and we use it for analytical studies. The reduction of
the \sigma-Hamiltonian to 3- and 1-band Hamiltonians is explicitly discussed
and we point out that, in addition to the hoppings commonly included in
many-body calculations, the 3-band Hamiltonian should include hopping between
all 2nd-nearest-neighbor oxygens and that the 1-band Hamiltonian should include
3rd-nearest-neighbor hoppings. We calculate the single-particle hopping between
the planes of a bi-layer. We show that the inclusion of t' is crucial for
understanding ARPES for the anti-ferromagnetic insulator Sr_2CuO_2Cl_2.
Finally, we estimate the value of the inter-plane exchange constant for an
un-doped bi-layer in mean-field theory using different single-particle
Hamiltonians.
