We show that the incidental numerical instabilities in explicitly correlated coupled cluster R 12 (CC-R 12) calculations can be a priori removed by using systematically developed R12 suited basis sets. Those are quite different from basis sets developed for conventional calculations mainly in polarization functions. Using even tempered polarization sets for carbon and oxygen we were able to define compromise R12 suited bases which describe equally well atomic ground states, positive and negative ions and electric field polarized atoms. Corresponding energies obtained with such R12 universal' sets remain accurate to within a few muE(h) with respect to the R12 optimal' bases of individual systems. (C) 2002 Elsevier Science B.V. All rights reserved.