Explicitly correlated R12 coupled cluster calculations for open shell systems

Explicitly correlated coupled cluster R12 (CC-R12) calculations for open shell systems are reported for the first time. We have implemented a single reference UHF based CC-R12 at the level of single and double excitations (CCSD-R12). Illustrative results for first row atoms and their anions are shown, together with electron affinities. We have turned our attention to convergence properties toward the basis set limit using various modifications of the 'standard approximation'. It is shown that for the vast majority of calculations one can skip some of the computationally demanding terms, basically without loss of accuracy. (C) 2000 Elsevier Science B.V. All rights reserved.