Coupled cluster
Coupled cluster method is a technique used for description of the many-body systems. The method was initially developed by Fritz Coester and Hermann Kümmel in 1950's for studying nuclear physics phenomena but it became more frequently used after Jiři Číek and Josef Paldus reformulated the method for studying electronic correlation in atoms and molecules in 1960's. It is now one of the most prevalent methods in quantum chemistry that include electronic correlation. The method is based on exponential Ansatz:
 ,
where  is the wave function  is the reference function (e.g. Hartree-Fock function), and  is the cluster operator:
 ,
where in the formalism of second quantization:
In the above formulae  and  denote the creation and anihilation operators and i,j stand for occupied and a,b for unoccupied orbitals.
In the simplest version one considers only  operator (double excitations). This method is called coupled cluster with doubles (CCD in short).
The coupled cluster equations are usually derived using diagrammatic technique and result in nonlinear equations which can be solved in an iterative way.
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