Journal of Physics B: Atomic, Molecular and Optical Physics
General methods for evaluating three-electron integrals in Hylleraas coordinates are given. Formulae are obtained for the matrix elements of various operators arising in Hylleraas-type variational calculations for states of arbitrary angular momentum. For the calculations of Breit interaction, a number of reduction relations are developed for the elimination of singularities in some singular integrals. A numerically stable scheme is presented for the case when one of the powers of r ij is -2.
Yan, Z. C. and Drake, Gordon W. F.. (1997). Computational methods for three-electron atomic systems in Hylleraas coordinates. Journal of Physics B: Atomic, Molecular and Optical Physics, 30 (21), 4723-4750.