Perturbation theory summation over excited states

Various forms of Hartree-Fock perturbation theory have been applied to the calculation of nitrogen screening constants within the framework of Ramsey s treatment. These avoid the difficulties associated with the infinite summation over excited states by restricting the summation to states described by the molecular orbitals used in the calculation. (11-14)... 

Formalism of the so-called response theory is another, quite universal language for the description of the more general approach to Rayleigh-Schrodinger perturbation theory suggested above, in which the summation over excited states is effectively replaced with solving a large system of linear equa-... 

Since in these formulae summation over all excited electronic states occurs, the present form of the perturbation theory used to be termed the sum-overstates perturbation theory. 

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