Average energy denominator method

The infinite sum may be avoided by means of the closure method 2 I >< Pm = 1. An average energy is chosen for all the excited states, which enables the polarizability to be written in terms of a constant denominator and matrix elements for the ground-state wavefunction only. This is only a rough approximation which, though widely applied in approximate calculations, is of little use for more than an order of magnitude estimate of a. 

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