Non-iterative corrections to extended coupled-cluster energies employing the generalized method of moments of coupled-cluster equations

It is shown that the extended coupled-cluster method with singles and doubles ( ECCSD) does not suffer from the non-variational collapse observed in the standard CCSD calculations when multiple bond breaking is examined. This interesting feature of the single-reference ECCSD theory is used to design the non-iterative CC methods with singles, doubles and non-iterative triples and quadruples, which provide a highly accurate and variational description of potential energy surfaces involving multiple bond breaking with computational steps that scale as N-6-N-7 with the system size. This is accomplished with the help of the generalized version of the method of moments of coupled-cluster equations ( GMMCC), which can be used to correct the results of non-standard CC calculations, such as ECCSD. The theoretical considerations are illustrated by the preliminary results of the ECCSD-based GMMCC calculations for triple bond breaking in N-2.