Spin-free [2]_R12 basis set incompleteness correction to the local multireference configuration interaction and the local multireference average coupled pair functional methods

The local multireference configuration interaction (LMRCI) and local multireference averaged coupled pair functional (LMRACPF) methods are extended to include explicit correlation via the universal spin-free [2]_R12 basis set incompleteness correction. Four test cases are examined to measure the performance of the LMRCI+[2]_R12 (without and with the Davidson + Q correction for size-extensivity) and LMRACPF+[2]_R12 methods. These tests examine bond dissociation energies (BDEs) for ethene, perfluoroethene, propene, and 2-butene. As has been demonstrated for other methods, the LMRCI+[2]_R12/LMRCI+Q+[2]_R12/LMRACPF+[2]_R12 BDEs are as accurate as the conventional LMRCI/LMRACPF BDEs that are computed with the basis set one cardinal number higher. It is shown that LMRCI+[2]_R12/LMRCI+Q+[2]_R12/LMRACPF+[2]_R12 BDEs computed with the June calendar basis sets preserve the accuracy of the corresponding BDEs computed with the conventional aug-cc-pVXZ basis sets (where X = D, T, Q).