QCCD method

However, there is an interesting intermediate step between standard CCD and ECCD. This is a model that includes terms quadratic in A2 in the bra function. We term this energy functional the quadratic coupled cluster doubles (QCCD) method (79) ... 

As in the recent QCCD study by Head-Gordon et al. (28, 128), we tested the ECCSD, LECCSD, and QECCSD methods, based on eqs (52)-(59), using the minimum basis set STO-3G (145) model of N2. In all correlated calculations, the lowest two core orbitals were kept frozen. As in the earlier section, our discussion of the results focuses on the bond breaking region, where the standard CCSD approach displays, using a phrase borrowed from ref 128, a colossal failure (see Table II and Figure 2). 

The QCCD energy is to be made stationary with respect to both the ket (T2) and bra (A2) degrees of freedom, as well as the orbitals. In this respect it is a generalization of the conventional optimized orbital CCD method (77). However, the extra term relative to CCD introduces new physics, such as the coupling of the A2 and T2 amplitudes. For example, the equations for the T2 amplitudes can be symbolically written as ... 

---