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Computational materials design often profits from the fact that some complicated contributions are not calculated for the real material, but replaced by results of models. We turn this approximation into a very general and in principle exact theory by introducing the concept of a connector, which is a prescription of how to use the results of a model system in order to simulate a real system. We set the conditions that must be fulfilled for the existence of an exact connector. We demonstrate that, and why, this approach is a very convenient starting point for approximations. We also show that the connector theory can be used to design new functionals, for example for density functional theory. We illustrate our purposes with simple but realistic examples.
A method is developed to calculate the ligand field (LF) parameters and the multiplet spectra of local magnetic centers with open $d$- and $f$-shells in solids in a parameter-free way. This method proceeds from density functional theory and employs W
We present a perturbative model for crystal-field calculations, which keeps into account the possible mixing of states labelled by different quantum number J. Analytical J-mixing results are obtained for a Hamiltonian of cubic symmetry and used to in
We study the electron-hole pair (or excitonic) condensation in the extended Falicov-Kimball model at finite temperatures based on the cluster mean-field-theory approach, where we make the grand canonical exact-diagonalization analysis of small cluste
In brittle fracture applications, failure paths, regions where the failure occurs and damage statistics, are some of the key quantities of interest (QoI). High-fidelity models for brittle failure that accurately predict these QoI exist but are highly
A versatile method for combining density functional theory (DFT) in the local density approximation (LDA) with dynamical mean-field theory (DMFT) is presented. Starting from a general basis-independent formulation, we use Wannier functions as an inte