Closed shell CCSD method

Today, analytical gradients for CC methods have been accomplished for CCD [116], and by using the above relaxed density formulation, closed-shell CCSD [117] and CCSDT-1 [118]. Also for CCSD(T) [119,120]. General, open-shell, symmetry-specific CCSD [113], and CCSD(T) have been presented and are widely used in the ACES II program system [121,122]. Now analytical derivatives for the full CCSDT are available... 

Second, where nondynamical correlation is unimportant (that is, mainly closed-shell molecules near their equilibrium geometries), size-extensive (or nearly size-extensive) treatments like CPF, CCSD, and CISD+Q perform well. However, as bonds are stretched, or in species where nondynamical correlation is important because of near-degeneracies or open-shell effects, the performance of these methods deteriorates fairly rapidly. As might be expected, the CCSD method generally deteriorates slowest. In general, low-order perturbation theory methods do not agree well... 

Of all the methods currently used in molecular electronic structure theory, the CCSD(T) model is probably the most successful, highly accurate level, at least for closed-shell molecular systems. For many properties of interest to chemists such as molecular structure, atomization energies, and vibrational frequencies, it provides numerical data of consistently high quality, sometimes surpassing that of experiment. Nevertheless, it does fail in certain cases, in particular for systems characterized by several important Slater determinants and also for certain properties such as indirect nuclear spin-spin couplings of magnetic resonance spectroscopy. 

From Figs. 6 and 7, it may appear that there is little to be gained by going from MP2 (whose cost scales as K5) to CCSD (with cost K6). However, although the performance of MP2 and CCSD is quite similar for bond distances and reaction enthalpies of closed-shell systems of light atoms, for other properties and other systems, the more expensive and robust CCSD method outperforms MP2. 

The CCSD(T) approach is probably the most often exploited CC method in actual applications, and provides excellent results [145], as long as no quasi-degeneracy is present. This is usually the case for the closed-shell ground states near the equilibrium geometry. However, with the stretching of one or more genuine chemical bonds, the CCSD(T) method... 

For the computation of the interaction between two closed-shell monomers there are at present two excellent computational methods, both implemented in black box programs. The first is based on SAPT [135] and the second is the supermolecule CCSD method [136,137] with triply excited terms added in a non-iterative fashion. 

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