  Through a 2-dimensionnal tight-binding crystal orbital approach and a (CuO2)2
square unit cell of parameter a, we show that a Cu2+/O2- -> Cu+/O- charge
transfer is likely to occur at the M($\pi/a$, $\pi/a$) point of the Brillouin
zone, for O4 groups with antibonding b1g symmetry. This approach emphasises the
role of oxygen-oxygen interactions in avoiding the nesting of the Fermi surface
and agrees with its observed topology. At the M point of the Brillouin zone,
oxygen atoms are strongly dissymmetric ("Janus" atoms) and link copper atoms
with different environments (a1g vs. b1g symmetries). Further hole doping
generates two situations: two holes (S=0) and/or a single hole (S=+/- 1/2) in
the O4 b1g groups ($\sigma$ or $\pi$), with a possible equilibrium between
them; the former can be considered as a "hole lone pair" by analogy with
electron lone pairs. Mulliken-Jaffe electronegativity considerations justify
the nearly zero U Hubbard parameter value. The mixing of the pair-occupied with
pair-iunoccupied wave functions is realised via an electronic hamiltonian in
place of the electron-phonon coupling of the pristine BCS theory.
