Darwin operator Cowan-Griffin

Only recently has the work of Bauschlicher, Walch and Siegbahn showed the need to include d correlation, whereas the work of Werner and Martin and of Scharf, Brode and Ahlrichs stressed the importance of cluster corrections and relativistic corrections. The results of Werner et al. were obtained using CEPA-1 to account for cluster contributions, while Scharf et al. used the CPF approach. Both groups accounted for relativistic corrections by employing first-order perturbation theory, i.e. by evaluating the Cowan-Griffin operator which consists of the mass-velocity and the one-electron Darwin term of the Breit-Pauli Hamiltonian. 

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