A variational approach to two-body correlations in atoms

A variational method for calculating the correlation structure of atoms is proposed. The full wavefunction is written in the form PSI=ΩPHI where PHI is a sum of determinantal functions. Ω is expanded in terms of one-, two-, three- etc body cluster operators. The following approximations are made: (i) the cluster expansion is truncated; (ii) the cluster operators are taken to be local or to involve bilinearly spin, orbital angular momentum and linear momentum operators; and (iii) the cluster operators are expanded in terms of separable operators. The resultant expressions involve at most two-dimensional integrals, and the sums are tractable. Applications are discussed. (author)