Coupled cluster theory that takes care of the correlation cusp by inclusion of linear terms in the interelectronic coordinates

CC-R12—a combination of coupled cluster theory and the R12 method, is presented in which the correlation cusp is treated via inclusion of terms explicitly dependent on the interelectronic distance rij into the exponential expansion of the wave function. A diagrammatic derivation of the CC-R12 equations within the so-called ‘‘standard approximation B’’ is given at the level of singles, doubles and triples (CCSDT-R12). MBPT(4)-R12 is derived as a byproduct of CCSDT-R12. Fifth order noniterative corrections are also discussed.