Multi-gap absorption in CaCu$_{3}$Ti$_{4}$O$_{12}$ and the predictivity   of ab initio methods
  We report the electronic properties of the quadruple perovskite CaCu$_{3}$Ti$_{4}$O$_{12}$ as obtained via several density-functional based methods, and propose a new interpretation of optical experiments to the effect that four distinct transitions (centered around 0.7, 1.5, 2.5, and 3.5 eV) contribute to the spectrum. The comparison with experiment is satisfactory, especially after we account for the effects of spin disorder, which does not close the fundamental gap but suppresses the transition intensity. We find that some of the methods we employ tend to overestimate considerably the gaps for standard values of the respective adjustable parameters. 
Francesco Ricci, Paola Alippi, Alessio Filippetti, and Vincenzo   Fiorentini
cond-mat.mtrl-sci