Configuration interaction size consistency

A disadvantage of all these limited Cl variants is that they are not size-consistent.The Quadratic Configuration Interaction (QCI) method was developed to correct this deficiency. The QCISD method adds terms to CISD to restore size consistency. QCISD also accounts for some correlation effects to infinite order. QCISD(T) adds triple substitutions to QCISD, providing even greater accuracy. Similarly, QCISD(TQ) adds both triples and quadruples from the full Cl expansion to QCISD. 

In comparison with the more standard Configuration Interaction (Cl) method, the one-particle Green s function approach offers the essential advantages, in the outlook of numerical applications on extended systems, of a stronger and systematic compactness (30) of the configuration spaces in high order approximations and of energy separability (5,31) in the dissociation limit (size-consistency). The latter is a necessary prerequisite ( ) for a correct (i.e. size-... 

The electron correlation problem remains a central research area for quantum chemists, as its solution would provide the exact energies for arbitrary systems. Today there exist many procedures for calculating the electron correlation energy (/), none of which, unfortunately, is both robust and computationally inexpensive. Configuration interaction (Cl) methods provide a conceptually simple route to correlation energies and a full Cl calculation will provide exact energies but only at prohibitive computational cost as it scales factorially with the number of basis functions, N. Truncated Cl methods such as CISD (A cost) are more computationally feasible but can still only be used for small systems and are neither size consistent nor size extensive. Coupled cluster... 

In this work we are concerned with the dressing method known as Size-Consistent Self-consistent Singles and Doubles Configuration Interaction, usually abbrebiated to (SC) SDCI [8]. This method can be considered as a generalization of the CEPA-3 formalism, as has been shown elsewhere [8,35]. It provides size-extensive roots and... 

Size consistency is a more important attribute than variational, and because of this, Moller-Plesset models are generally preferred over configuration interaction models. 

Size Consistent. Methods for which the total error in the calculated energy is more or less proportional to the (molecular) size. Hartree-Fock and Moller-Plesset models are size consistent, while Density Functional Models, (limited) Configuration Interaction Models and Semi-Empirical Models are not size consistent. 

Let s compare these CASSCF results with those for a method that is not size-consistent, Cl with no complete aspect. We ll use CISD (configuration interaction singles and doubles Section 5.4.3). Here are the results for CISD/6-31G ... 

Meissner L (1988) Size-consistency corrections for configuration interaction calculations. Chem Phys Lett 146 204-210... 

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