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Tight-binding fits of the $pi$-bands. The strong VO
R
coupling splits the bands into bonding and antibonding groups. The bonding bands cross the Fermi level. The next in strength VO
L
coupling yields the dispersion along the ladders. The tiny VV coupling gives the dispersion perpendicular to the ladders. The antibonding bands of the full band structure can only be fitted by including direct O-O couplings: O
R
O
L
and O
R
O
R
remove the dispersion from the antibonding bands, and O
L
O
L
adds to the bonding-antibonding splitting. The molecular character of the V-orbital then adds naturally a VO
R
coupling to the next nearest neighbor. These additional couplings have little effect on the bonding bands of the homogeneous phase, become, however, crucial in those charge ordered situations, where the hopping along the ladder legs is supressed.
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