Size-extensivity error

It should be noted, however, that for a fixed system size, increasing the number of references decreases size extensivity errors. Indeed, for very highly correlated MR-CI wavefunctions, applying corrections such as (54) and (55) can sometimes lead to less reliable results. 

For avoiding the intruder state problem in BWCCSD we pay the price of losing size extensivity. Unfortunately, for the purpose of chemical application of MR BWCCSD, the size-extensivity error is not tolerable. For example, the MR BWCCSD value of the singlet-triplet gap in twisted ethylene exhibits an error of 3 kcal/mol with respect to full Cl [40]. Even worse case is the dissociation of the F2 molecule, where MR BWCCSD... 

Extent of the size-extensivity error depends on several factors type of the resolvent, active space, derivation of working equations for amplitudes and the presence or absence of disconnected diagrams in the diagrammatic representation of working equations. In Table 18.1 we show some examples. 

Elimination of the size-extensivity error in the MRBWCC theory was suggested by Hubac and Wilson [68]. The Brillouin-Wigner perturbation theory has been known notoriously as a method not furnishing size extensivity. There was therefore every reason to believe that the size-extensivity error in MR BWCCSD originated from the use of the Brillouin-Wigner type resolvent (15) instead of the Rayleigh-Schrodinger... 

In their 1999 paper[26], Sekino and Bartlett defined several models to ameliorate the size-extensivity error of the EOM-CC approach, while simultaneously avoiding the evaluation of the expensive quadratic term in the full second-order CCLR expression in Eq. (21). One of these, dubbed Model III, eliminated the size-extensivity error of EOM-CC completely by (a) retaining only the term linear in T in Eq. (21), (b) dropping the commutator. 

Keywords Block correlated coupled cluster method energy surface Size-extensivity error... 

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