Connector theory for reusing model results to determine materials properties

The success of Density Functional Theory(DFT)is partly due to that of simple approximations,such as the Local Density Approximation(LDA),which uses results of a model,the homogeneous electron gas,to simulate exchange-correlation effects in real materials.We turn this intuitive approximation into a general and in principle exact theory by introducing the concept of a connector:a prescription how to use results of a model system in order to simulate a given quantity in a real system.In this framework,the LDA can be understood as one particular approximation for a connector that is designed to link the exchange-correlation potentials in the real material to that of the model.Formulating the in principle exact connector equations allows us to go beyond the LDA in a systematic way.Moreover,connector theory is not bound to DFT,and it suggests approximations also for other functionals and other observables.We explain why this very general approach is indeed a convenient starting point for approximations.We illustrate our purposes with simple but pertinent examples.

Delocalization of dark and bright excitons in flat-band materials and the optical properties of V_(2)O_(5)

The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons,where electrons and holes stay close together,which is associated with a large binding energy.Here,using the example of the layered oxide V_(2)O_(5),we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but,at the same time,a large electron-hole distance,and we explain this seemingly contradictory finding.The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure,whereas the exciton extends orthogonally to the chains formed by the crystal structure.Moreover,we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy.These findings are obtained by combining first principles many-body perturbation theory calculations,ellipsometry experiments,and tight binding modelling,leading to very good agreement and a consistent picture.Our explanation is general and can be extended to other materials.