WHAT CAN BE LEARNED ABOUT H IGH Tc FROM LOCAL DENSITY THEORY?

The significance of local density band structure results for high Tc compounds is critically discussed. It is pointed out that straightforward application of this method can be misleading because of the correlated nature of these materials. However, with LDA numbers can be derived for the parameters appearing in the models in which correlation is treated ezplicitely. In this way we arrive at the conclusion that despite a large U the high Tc materials hould be viewed as itinerant materials. Turning to ground state properties, we show that the LDA /ails to describe the antiferromagnetism o/La2Cu04 and it is pointed out that this may be cured by the inclusion of self interaction corrections. Finally, using a proposed ozygen defect structure for YBa2CusOT-z and a simple tight binding model based on band structure calculations, we show that jumps and plateauz in the hole counts in the planes and chains occur as a f~nction of x. These features are perfectly correlated with the occurrence of the antiferromagnetic, as well as of the 60K and 90K superconducting phases. 1