We show that the long-range part of the Coulomb self-energy of an elec
tron density rho(r) is almost identical to that of its associated Stew
art density <(rho)over tilde>(r). Since the latter is obtained by fitt
ing rho(r) within an auxiliary basis of only s functions, our proposal
can be viewed as a hybrid of the RI (resolution of the identity) and
KWIK methods. (C) 1998 Elsevier Science B.V.