Electronic Structure and Fermiology of High-T$\text{}_{c}$ Superconductors

We briefly discuss some of our recent results in three areas, all related to issues concerning the electronic structure and fermiology of the high-T$\text{}_{c}$'s. (i) First-principles band theory based computations of the electron-positron momentum density relevant for the interpretation of the positron annihilation (2D-ACAR) experiments; results for YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7}$ are discussed. (ii) Another k-resolved spectroscopy where such first-principles comparisons between the computed and measured spectral intensities have been crucially important is the angle-resolved photoemission experiment; we discuss results for the (001)-surface of YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7}$ where computations yield considerable insight into the nature and origin of the spectral signal associated with the CuO$\text{}_{2}$ plane bands. (iii) The study of effects of substitutions and of oxygen-vacancy disorder in the high-T$\text{}_{c}$'s; here results for the LaSrCuO$\text{}_{4}$, and Ba-K-Bi-O and Ba-Pb-Bi-O systems, with focus on the nature of the van Hove singularity in the vicinity of the Fermi energy are presented.