Strongly Correlated Systems

Soon after the discovery of high Tc superconductors a new class of methods for strongly correlated materials, the so-called LDA+U, was introduced to overcome the inadequacies of the single-particle picture in DFT. Recently the rotationally invariant version of this method has been implemented in the eclectic plane-wave-pseudopotential method for electronic structure calculations. This was realized by introducing localized d level occupation matrices as projections of the occupied electronic manifold on suitable atomic states. In this implementation the exchange contribution to the effective electronic interaction is neglected and a direct connection between the correction term with the spurious electronic self-energy present in DFT calculations is made. The method is non-empirical. The needed parameter U is determined by studying the response of the system under consideration to localized perturbations of the Hubbard ions and properly extracting from the total response function to the local interaction term. The great advantage of a plane-wave implementation of the LDA+U method is the easiness in obtaining Hellmann-Feynman forces and quantum mechanical stresses, that enable structural relaxations and MD simulations, as well as phonon calculations, thus giving access to free energy calculations within the quasi-harmonic approximation. This is the method we will use to explore for Mg/Fe silicate alloys. It has been successfully used in the study of oxygen vacancies in ceria (cerium oxide) where LDA+U approach allows to treat on the same ground the different valence states of cerium occurring for different stoichiometries. (Wentzcovich, de Gironcoli, and Baroni)



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