Electron correlations in multiconfiguration atomic wave functions.

A two-electron spin-independent operator is defined to represent the spatial correlation of the electrons in a general atom or ion. The spherical-tensor expansion of this operator provides a systematic method for evaluating the two-electron correlation function implied by a Hartree-Fock N-electron wave function including arbitrary configuration mixing. The method is illustrated by examples involving electron coalescence in helium and carbon, and core polarization in sodium. This technique for computing the electron correlation function provides a useful tool of wide applicability in comparing different theoretical approaches to the structure of complex atoms. @S1050-2947~96!06306-8#