The multi-configuration frozen core approximation. I. Eigenstates of the two-electron atom

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Abstract
This paper describes a generalization of the 'frozen core' approximation in which several configurations are employed, each containing a fixed core function chosen where possible from physical considerations. This multiconfiguration frozen core (mcfc) procedure is applied to the ground 1s states of he and h- and to the singly excited 1s, 3s, 1p, 3p, 1d and 3d states of he (up to principal quantum number n=7), two of the core functions being chosen so as to take account of the dipole and quadrupole components of the long-range correlation. Eigenenergies of high accuracy are obtained using at the most five core functions: expectation values of several operators are also calculated. The success of a special case of the mcfc method, the generalized polarized orbital approximation, confirms in a striking way that short-range correlation is of little importance in all except the 1s series. It is concluded that for many purposes the mcfc method may possess some distinct advantages when compared with other configuration interaction procedures.