In this dissertation we have developed and applied the state specific multireference coupled cluster theory suggested by Mukherjee and co workers (Mk-MRCC). For model systems, Mk-MRCC is found to provide more accurate results than competitor theories at all levels of truncation of the cluster operator. The first production level code for Mukherjee multireference coupled cluster singles and doubles (Mk-MRCCSD) computations has been written. A crucial element for the development of Mk-MRCC into a computational tool was the realization that the coupling terms appearing in the equations could be written in a simple closed form. Mk-MRCCSD results are reported for the singlet triplet splittings in ortho-, meta-, and para-benzyne, coming within 1.5 kcal/mol of experiment in all cases. We also report the first implementation with correct scaling of the Mk-MRCC method with singles, doubles, and approximate iterative triples Mk-MRCCSDT-n, (n = 1a,1b,2,3) as well as full triples (Mk-MRCCSDT). In all model systems the various Mk-MRCCSDT-n approaches recover on average between 59% and 73% of the full triples effect.