Explicitly correlated internally contracted multireference coupled-cluster singles and doubles theory: ic-MRCCSD(F12*)

An explicitly correlated ansatz employing Slater-type geminals and cusp conditions is developed for the internally contracted multireference coupled-cluster singles and doubles method. Only the most important geminal terms are retained in the spirit of earlier work for single-reference theory. Throughout all our test calculations, the new ic-MRCCSD(F12*) method improves the basis set convergence of many properties, e. g., spectroscopic constants or singlet-triplet splittings, with only little extra computational cost. If a perturbative correction for connected triples is included (the ic-MRCCSD(F12*)+(T) method), very accurate results can be obtained even with minimal active spaces. (C) 2013 Elsevier B. V. All rights reserved.