Epstein-Nesbet type

Dabcd CCSD), respectively, whereas the CR-CC(2,3) and other CR-CCOm/i. mg) schemes discussed here rely on the Epstein-Nesbet-type denominators, such as 7) (CCSD), although we could obviously consider alternative forms of these denominators as well, as implied by the above considerations. Just like CCSD(2)y and CCSD(2), the CR-CC(2,3) and CR-CC(2,4) methods are rigorously size extensive. This has been illustrated numerically in Ref. [46]. 

CCSD(2) theory of Gwaltney and Head-Gordon [20, 21]. If we approximate the diagonal even further and replace the Epstein-Nesbet-type... 

In addition to our earlier work (9,54), we wish to point out that we can also calculate the first-order coefficients by relying entirely on perturbation theory. Of course, the result will very much depend on the way we partition the Hamiltonian H into the unperturbed part Hq and the perturbation W, H -Hq+W. Since our zero-order wave function is assumed to represent a general multi-configurational Cl wave function, it is easier to employ the Epstein-Nesbet (EN) type perturbation theory. For this purpose we choose the unperturbed Hamiltonian Hq as follow... 

---