Explicitly correlated R12 coupled cluster calculations for open shell systems

Explicitly correlated coupled cluster R12 (CC-R12) calculations for open sh ell systems are reported for the first time. We have implemented a single r eference UHF based CC-R12 at the level of single and double excitations (CC SD-R12). Illustrative results for first row atoms and their anions are show n, together with electron affinities. We have turned our attention to conve rgence properties toward the basis set limit using various modifications of the 'standard approximation'. It is shown that for the vast majority of ca lculations one can skip some of the computationally demanding terms, basica lly without loss of accuracy. (C) 2000 Elsevier Science B.V. All rights res erved.