Physical Review A
Expectation values of rP for an arbitrary nl hydrogenic state are expressed in terms of state-independent coefficients that are determined by a simple recursion relation. Explicit results are given for negative P down to -16 and, by an algebraic transformation, for positive P up to 13. Closed-form expressions are obtained for the coefficients of the two highest-order terms in l multiplying each power of n. The result is an asymptotic expression valid for each P and sufficiently large l. © 1990 The American Physical Society.
Drake, Gordon W. F. and Swainson, R. A.. (1990). Expectation values of rP for arbitrary hydrogenic states. Physical Review A, 42 (3), 1123-1126.