Abstract
The internally contracted multireference coupled$\backslash$ncluster (icMRCC) method allows a highly accurate$\backslash$ndescription of both static and dynamic correlation$\backslash$nwith a computational scaling similar to single$\backslash$nreference coupled cluster theory. The authors show$\backslash$nthat the method can lose its orbital invariance and$\backslash$nsize consistency when no special care is taken in$\backslash$nthe elimination of redundant excitations. Using the$\backslash$nBeH₂ model system, four schemes are compared which$\backslash$ndiffer in their treatment of linear dependencies$\backslash$nbetween excitations of different rank (such as$\backslash$nbetween singles and doubles). While the energy$\backslash$ncurves agree within tens of μEh when truncating$\backslash$nthe cluster operator at double excitations$\backslash$n(icMRCCSD), inclusion of triple excitations$\backslash$n(icMRCCSDT) leads to significant differences of more$\backslash$nthan 1 mEh. One scheme clearly yields the best$\backslash$nresults, while the others even turn out to be not$\backslash$nsize consistent. The former procedure uses genuine$\backslash$nsingle and double excitations and discards those$\backslash$nlinear combinations of (spectator) double and triple$\backslash$nexcitations which have the same effect on the$\backslash$nreference function. With this approach, the$\backslash$nequilibrium structure and harmonic vibrational$\backslash$nfrequencies of ozone obtained with icMRCCSDT are in$\backslash$nexcellent agreement with CCSDTQ. The authors further$\backslash$napply icMRCC methods to potential energy surfaces of$\backslash$nHF, LiF, N2, and to the singlet-triplet splitting of$\backslash$nbenzynes. In particular, the latter calculations$\backslash$nhave been made possible by implementing the method$\backslash$nwith the proper formal scaling using automated$\backslash$ntechniques.
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