Full configuration interaction description

A CASSCF calculation is a combination of an SCF computation with a full Configuration Interaction calculation involving a subset of the orbitals. The orbitals involved in the Cl are known as the active space. In this way, the CASSCF method optimizes the orbitals appropriately for the excited state. In contrast, the Cl-Singles method uses SCF orbitals for the excited state. Since Hartree-Fock orbitals are biased toward the ground state, a CASSCF description of the excited state electronic configuration is often an improvement. 

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. 

As mentioned in section 1, the combination of the CI method and semiempirical Hamiltonians is an attractive method for calculations of excited states of large organic systems. However, some of the variants of the CI ansatz are not in practical use for large molecules even at the semiempirical level. In particular, this holds for full configuration interaction method (FCI). The truncated CI expansions suffer from several problems like the lack of size-consistency, and violation of Hellmann-Feynman theorem. Additionally, the calculations of NLO properties bring the problem of minimal level of excitation in CI expansion neccessary for the coirect description of electrical response calculated within the SOS formalism. 

MNDOC is a correlated version of MNDO. Unlike all previously discussed methods, MNDOC includes electron correlation explicitly and thus differs from MNDO at the level of the underlying quantum chemical approach (a) while being completely analogous to MNDO in all other aspects (b)-(d) except for the actual values of the parameters. In MNDOC electron correlation is treated conceptually by full configuration interaction, and practically by second-order perturbation theory in simple cases (e.g., closed-shell ground states) and by a variation-perturbation treatment in more complicated cases (e.g., electronically excited states).The MNDOC parameters have been determined at the correlated level and should thus be appropriate in all MNDO-type applications which require an explicit correlation treatment for a qualitatively suitable zero-order description. In closed-shell ground states... 

Let us now consider the description of an array of well-separated systems afforded by the full configuration interaction model. As in Section 3.1, we shall again restrict... 

The import of diabatic electronic states for dynamical treatments of conical intersecting BO potential energy surfaces is well acknowledged. This intersection is characterized by the non-existence of symmetry element determining its location in nuclear space [25]. This problem is absent in the GED approach. Because the symmetries of the cis and trans conformer are irreducible to each other, a regularization method without a correct reaction coordinate does not make sense. The slope at the (conic) intersection is well defined in the GED scheme. Observe, however, that for closed shell structures, the direct coupling of both states is zero. A configuration interaction is necessary to obtain an appropriate description in other words, correlation states such as diradical ones and the full excited BB state in the AA local minimum cannot be left out the scheme. 

All different MC or empirical indices of the full normal (non semi-random) descriptors of Tables 7.9 and 7.10 are now joined together to form a new space of indices, kind of super-indices, which will be used for a full combinatorial search of the best super-descriptors in a kind of configuration interaction of best indices. This super-descriptor space gave no remarkable results with the greedy algorithm, but it does find improved descriptions for the following four properties. 

In Chapter 11, we treat configuration-interaction (Cl) theory, concentrating on the full Cl wave function and certain classes of truncated Cl wave functions. The simplicity of the Cl model allows for efficient methods of optimization, as discussed in this chapter. However, we also consider the chief shortcomings of the Cl method - namely, the lack of compactness in the description and the loss of size-extensivity that occurs upon truncation of the Cl expansion. 

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