deletions | additions       diff --git a/introduction.tex b/introduction.tex  index 2994f4d..5103009 100644  --- a/introduction.tex  +++ b/introduction.tex 
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Here $\mu $ is the chemical potential and we have singled out in Eq.~\ref{eq:gwk} the Hartree potential expressed in terms of the exact density and the crystal potential, and  lumped the rest of the effects of the correlation in the self energy operator which depends on frequency as well as on two space variables.   In chemistry, a quantum mechanical system is strongly correlated when $\Sigma( i \omega) - \Sigma_{HF}(i \omega)  $ is large at small energies. Here $\Sigma_{HF}(i omega)  $ is the self energy computed in the Hartree Fock approximation.NOTE  At infinite frequency, $\Sigma $ is given by the Hartree Fock graph evaluated with the exact Greens function. function, hence the Hartree Fock approximation is not exact even at infinite frequency, but it is a good starting point for the treatment of atoms and molecules.  Solid state physicists adopt a very different definition of strong correlations. Here, the a good  reference system is the Kohn Sham Greens function evaluated in some implementation of the density functional theory such as the LDA. Hence, for condensed matter scientists, by definition, a  strongly correlated material is one where $\Sigma - V_{KS} $ is large at low energy. frequencies. Strongly correlated materials, are those for which this is not the case, a famous example are materials such as LaCuO4 which are predicted to be metals in LDA but which are experimentally antiferromagnetic Mott insulators.  In quantum chemistry   Aron J Cohen, Paula Mori-Sánchez, Weitao Yang 
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Pages  792-794  there is a classification of the errors introduced by the use of  approximate density functionals, as being of two types, static correlation and dynamic correlation. These correlation\cite{Yang_2012}. Similar  ideas,have  also been reformulated appear  in the solid state context. Unfortunately context, but  the terms are nomenclature is  exchanged. A {\it static } self energy (i.e. a self energy which varies weakly with frequency at low energies) correspondes corresponds  to the concept of dynamical correlations.