Perturbative Configuration Interaction with Localized Orbitals

Perturbative Configuration Interactions with Localized Orbitals structure-activity relationships endo-2-aminobenzobicyclo[2.2.1]— heptene... 

PCILO perturbed configuration interaction with localized orbitals... 

The most extensive and systematic study of the chemistry of lignin with theoretical methods has been performed by Remko and co-workers. Their work has involved the nature of intramolecular (40-43) and inter-molecular (44-48) hydrogen bonding of lignin model compounds, spectral transitions (49-52), and conformational analysis (53). The methods used have included CNDO/2 and PCILO (Perturbative Configuration Interaction using Localized Orbitals) (54). 

An early example implementing the general approach to take into account first the intrabond correlation, is presented by the PCILO - perturbational configuration interaction of localized orbitals method [121,122], As one of its authors, J.-P. Malrieu mentions in [122], the PCILO method opposes the majority of the QC methods in all the fundamental concepts. In contrast to the majority of the methods based on the variational principle, the PCILO method is based on estimating the energy by perturbation theory. Also, the majority of the QC methods use one-electron HFR approximation, at least as an intermediate construct, whereas the PCILO is claimed to addresses directly the V-electron wave function and takes into account all surviving matrix elements of the electron-electron interactions. In contrast with other QC... 

All the above MO methods are referred to as semiempirical because the calculations make use of empirically derived constants chosen to allow the methods to agree with experiment or more rigorous MO calculations for some physicochemical properties of simple test molecules. The methods treat only the valence electrons in the molecule (such as four electrons in the 2s and 2p atomic orbitals of carbon). Another quantum mechanical method, which treats only valence electrons but is not an MO method, is PCILO (perturbative configuration interaction using localized orbitals) (Diner et al., 1969a,b Malrieu et al., 1969). The method is used mainly for conformational problems (rotations about bonds), and little use is ever made of the other quantities this method generates. One seldom-mentioned aspect in the use of PCILO is that lone pairs and double bonds must be explicitly located within the molecular structure, so that the predictions depend, for instance, on where the lone pairs of fluorine are put or where the double bonds of a substituted benzene are set. 

An alternative use of perturbation theory together with the idea of electron pairs is the method of Malrieu, Diner and Clavery abbreviated as PCILO ) (perturbative configuration interaction based on focalized orbitals). One uses a zeroth-order wave function, one in which each electron pair is in a fully localized LCAO-MO. All delocalization and correlation corrections are then treated by perturbation theory (not limited to second order). 

The parameterization of the exo-anomeric terms was based on the difference between the energy surfaces for 2-methoxytetrahydropyran (2) and 2-ethyltetrahydropyran (3) (see Figure 1), as calculated with the semi-empirical PCILO (perturbation of the configuration interaction matrix from localized bond orbitals) method (equations 7 and 8). ... 

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