Limited configuration interaction

Moser, C. M., and Lef bvre, R., J. Chem. Phys. 23, 598, "Lowest singlet excited levels of C10HS." Limited configurational interaction. 

It is possible to use full or limited configuration interaction wavefunctions to construct poles and residues of the electron propagator. However, in practical propagator calculations, generation of this intermediate information is avoided in favor of direct evaluation of electron binding energies and DOs. 

The idea of coupling variational and perturbational methods is nowadays gaining wider and wider acceptance in the quantum chemistry community. The background philosophy is to realize the best blend of a well-defined theoretical plateau provided by the application of the variational principle coupled to the computational efficiency of the perturbation techniques. [29-34]. In that sense, the aim of these approaches is to improve a limited Configuration Interaction (Cl) wavefunction by a perturbation treatment. 

Figure 4. One-dimensional MNDO isomerization pathways for tetra-methyl tetrahedrane and its radical cation. For the definition of the coordinate a cf. ( ) Cl denotes calculations including limited configuration interaction (see text).

We take the initial wavefunctions to be very limited configuration-interaction expansions, containing only s orbitals. Thus, we take as the first wavefunc-tion in the one formed by all single and double excitations generated from the basic configuration [Is ] by substitutions comprising the 2s orbital for the second wavefunction, we span over the 2s and 3s orbitals and for the third one, we use orbitals Is, 2s and 3s. Hence, are all these wavefunctions are full Cl wavefunctions for such bases. Explicitly, these wavefunctions are... 

CID Model. Configuration Interaction Doubles. A limited Configuration Interaction scheme in which only double excitations from occupied to unoccupied molecular orbitals are considered. 

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. 

Of the more exact methods, the limited configuration interaction (Cl MO LCAO) method and the self-consistent field (SCF MO LCAO) method will be mentioned. In contrast to the HMO method, both of these explicitly take electron repulsion into account. The Cl method is particularly valuable for the calculation of various physical properties, especially electronic spectra. A more detailed description is beyond the scope of the present review the reader is referred to original papers [Cl,17-20 SCF,21-23 and VESCF24 (variable electronegativity)] and to various reviews and monographs.5 25,26... 

Additional feature A limited configuration interaction calculation is... 

In the first place the Hiickel approximation for the tt electrons has been replaced by a self-consistent field (SCF) procedure, generally in a semiempirical approximation of the Pariser-Parr-Pople type completed with some limited configuration interaction (PPP-CI method).59 00 Second, the a skeleton of the molecules has been treated by the Del Re procedure61 for saturated systems (which is the counterpart for the a electrons of the Hiickel method for 77 electrons) as refined for the a skeletons of conjugated heterocycles by... 

The exact FCI (frill configuration interaction) solution of the PPP or Hubbard model is possible for molecules with up to about 16 atoms in the pi system. Any of the standard methods for performing approximate ab initio calculations, such as limited configuration interaction, Moeller-Plesset perturbation theory, or coupled cluster theory, may be applied to these models as well. All are expected to be very accurate at low order when U is small, but all will have to be pushed to higher order as U increases. 

The singlet insertion reaction was determined using the limited configuration interaction treatment outlined above.138 The singlet abstraction reaction... 

Flouquet and Horsley145 have recently published a study of the potential energy surface of the BlAi surface of HaO in the SCF approximation and with some limited configuration interaction. They found the form of the surface for this state... 

Electrocyclic Reactions.—Peyerimhoff, Buenker, and co-workers have carried out very detailed studies of the electrocyclic transformations159 between cyclic and open-chain hydrocarbons. The calculations employ a large GTO set of s- and p-type basis functions. In every study, the necessity of including limited configuration interaction was carefully investigated. The prototype electrocyclic transformation of cyclobutene to cis-butadiene via the thermochemical process has been studied in detail.160-161 The same authors also give an analysis of the qualitative theories for such reactions based on their ab initio calculations.163 A similar study of the electrocyclic transformations of cyclopropyl and allyl systems has also been made.163... 

Minimal Configuration Interaction.—A minimum requirement of a potential energy surface is that the wavefunction continuously describe the system as the reaction takes place, producing reactants and products in their correct states. In order for this situation to be obtained, the correlation error must either be taken fully into account or at the very least, be approximately constant over that portion of the potential energy surface under consideration. As discussed and illustrated in the previous section, one should at least perform a limited configuration interaction in which those configurations needed to provide the correct description of the dissociation products are included. 

Equations (6.123) are similar to the Roothaan-Hall equations used to obtain the Hartree-Fock energy. A full configuration interaction treatment is feasible only for the simplest molecular systems, and therefore much effort has been expended on establishing the best ways to achieve the optimum limited configuration interaction. One... 

R. A. Gangi and R. F. W. Bader, J. Chem. Phys., 55,5369 (1971). This calculation includes a very limited configuration interaction. 

K. Raghavachari and J. A. Pople, Int. j. Quantum Chem., 20, 1067 (1981). Calculation of One-Electron Properties Using Limited Configuration Interaction Techniques,... 

According to Figgis [5] one can consider a limited configuration interaction starting from the weak-field kets... 

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