The authors present evidence for the existence of two close-lying, nearly parallel energy bands in the vicinity of the Fermi level in YBa2Cu3O7. The electrons in these bands are located in the Ba-O planes and have nearly no Cu-O chain or plane components. The higher band is partially unoccupied and the corresponding Fermi surface was recently measured by the de Haas-van Alphen effect. They have calculated the extremal Fermi-surface areas and the results are in excellent agreement with experiment, when spin splitting in high magnetic field is taken into account. They show furthermore that interband transitions for in-plane polarizations are allowed between these two bands and that they result in a sharp peak at about 350 cm-1 in the calculated optical conductivity.
 

Physical Review B, 45 5103-6, 1992.