Full configuration interaction effect

Nondynamical electron correlation effects are generally important for reaction path calculations, when chemical bonds are broken and new bonds are formed. The multiconfiguration self-consistent field (MCSCF) method provides the appropriate description of these effects [25], In the last decade, the complete active space self-consistent field (CASSCF) method [26] has become the most widely employed MCSCF method. In the CASSCF method, a full configuration interaction (Cl) calculation is performed within a limited orbital space, the so-called active space. Thus all near degeneracy (nondynamical electron correlation) effects and orbital relaxation effects within the active space are treated at the variational level. A full-valence active space CASSCF calculation is expected to yield a qualitatively reliable description of excited-state PE surfaces. For larger systems, however, a full-valence active space CASSCF calculation quickly becomes intractable. 

The effectiveness of NSO s in reducing the expansion size in systems with more than two electrons is not as great and, in fact, for larger systems, their use is not practical. The loss in practicality is immediately obvious when one realizes that in order to obtain them, one must diagonalize the first-order density matrix of the exact wavefunction, i.e. a full configuration interaction must first be performed. Two methods have been introduced in order to regain the initial usefulness of natural orbitals the pseudonatural orbital method and the approximate or iterative natural orbital method. 

In principle, the theory reviewed in Sections 4-6 can be applied to interactions of arbitrary systems if the full configuration interaction (FCI) wave functions of the monomers are available, and if the matrix elements of H0 and V can be constructed in the space spanned by the products of the configuration state functions of the monomers. For the interactions of many-electron monomers the resulting perturbation equations are difficult to solve, however. A many-electron version of SAPT, which systematically treat the intramonomer correlation effects, offers a solution to this problem. 

Moszynski R, Jeziorski B, Szalewicz K (1994) Many-body theory of exchange effects in intermolecular interactions. Second-quantization approach and comparison with full configuration interaction results. J Chem Phys 100 1312-1325... 

To factor out basis set effects, we first compare in Table 24 full configuration interaction (FCI) excitation energies and dipole strengths (the values in parentheses) to those obtained with the TDA, RPA, ACISD, and EOM-CCSD approximations. Because FCI is a very expensive method, which has a factorial dependence on the number of electrons, we are restricted to a small system and present comparisons only for the beryllium atom and the CH" molecule." ... 

It is well known that the major deficiency of the Hartree-Fock model is its incapacity to account for the correlation effect associated with the motions of electrons of opposite spin. In principle, this contribution can be computed using a full configuration interaction (Cl) method, where the wavefunction corresponds to a variationally optimized combination of all possible electronic configurations. However, the application of this method to molecules of chemical interest can involve a number of configurations which rapidly... 

Full configuration interaction does scale linearly with the size of the system but is only computationally tractable for small systems described by small basis sets. In practice, the configuration interaction expansions must be truncated. Truncation is effected by including only states which are singly and doubly excited with respect to some reference configuration(s). Shavitt continues... 

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. 

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