Electron-pair extracule densities for low-lying excited states of He and Li+

摘要

The electron-pair extracule density is the probability density function for the position vector of the center-of-mass of an electron pair. Accurate calculations of spherically averaged extracule densities d(R) and their moments are reported for the 23S, 23P, and 21P states of the helium atom and lithium cation. The computations are based on explicitly correlated wave functions with energy errors of only 2-3 nanohartrees. The role of electron correlation is probed by comparison with Hartree-Fock calculations, and Fermi correlation is probed by the differences between the singlet and triplet P states.