Externally corrected coupled-cluster method

Keywords many-electron correlation problem, post-Hartree-Fock methods, coupled cluster approaches, configuration interaction, externally corrected coupled cluster methods, reduced multireference coupled cluster method... 

A different way to approximate SS-MRCC is the so-called externally corrected CCSD (ec-CCSD) [13-17], The ec-CCSD method is based on the Coupled Cluster Approach (CCA). In CCA the exact wavefunction is written in an exponential form,... 

Several possible choices of an external source have been tested so far. The basic requirement is that such a source must provide a reasonable approximation of the most important three- and four-body clusters that are missing in the SR CCSD approach. At the very least, we require it to describe the essential nondynamic correlation effects. The practical aspects require that it be easily accessible. The first attempts in this direction exploited the unrestricted Har-tree Fock (UHF) wave function [of different orbitals for different spins (DODS) type]. Its implicit exploitation lead to the so-called ACPQ (approximate coupled pairs with quadruples) method [26, 27]. Recently, its explicit version was also developed and implemented [31]. Although in many cases this source enables one to reach the correct dissociation channel, its main shortcoming is the fact that for the CS systems it can only provide T4 clusters, since the 7) contribution vanishes due to the spin symmetry of the DODS wave function. Nonetheless, the ACPQ method enabled an effective handling of extended linear systems (at the semi-empirical level), which are very demanding, since the standard CCSD method completely breaks down in this case [27]. 

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