Physical Review A
General formulas are obtained for the reduction of a wide class of two-electron matrix elements in Hylleraas coordinates to finite sums of radial integrals for states of arbitrary angular momentum. Multipole transition integrals and the Breit interaction are treated as special cases. A number of recurrence relations are derived for radial integrals containing PL(cosθ) (where cosθ=r^1•r^2) in terms of radial integrals containing lower-order Legendre polynomials. The results are well suited to computer implementation. © 1978 The American Physical Society.
Drake, Gordon W. F.. (1978). Angular integrals and radial recurrence relations for two-electron matrix elements in Hylleraas coordinates. Physical Review A, 18 (3), 820-826.