Range of Electron Correlation in the Helium Atom

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Abstract
For the helium atom, a new type of correlation factor is tested which has the form 1+ch(r12) where h(r12) is a cutoff correlation variable such that h(r12) = r12 for 0≤r12≤D, and h(r12) = D for D≤r12≤∞, with c and D adjustable parameters. In a series of energy calculations using wavefunctions which contain both configuration interaction and cutoff r12 terms, it is found that, as far as the energy is concerned: (1) The correlation cusp is unimportant, and (2) The range of the Coulomb correlation effect is of the order of the atomic diameter. For very small D, the correlation energy per unit volume is calculated to be 0.048 hartree/bohr3.