Perturbative Configuration Interaction

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... 

Perturbative Configuration Interaction over Localized Orbitals Pseudo-natural Orbital or (PSNO)... 

In this study, we have chosen the supermolecule approach and have used the semi-empirical quantum mechanical method called PCILO (Perturbative Configuration Interaction using Localized Orbitals) (16) to calculate intermolecular interactions. This method has recently been used successfully to calculate the intermolecular energies and geometries of hydrogen-bonded dimers of hydrocarbons and water (17,18). H-bonded complexes are particularly well characterized by this method (19). 

Kaufman, Joyce J., "A Suggested Procedure to Improve the Description of Lone Pairs in the PCIL0 or More General Ab-Initio Perturbative Configuration Interaction Schemes Based... 

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

PCILO perturbed configuration interaction with localized orbitals... 

The methodology of a direct evaluation of the exchange coupling constant is best demonstrated when the two-orbital model is reformulated the MO calculation is done first, then a transformation to the orthogonalised magnetic orbitals is applied and finally a perturbative configuration interaction is used. 

The second-order perturbation configuration interaction brings a correction... 

The perturbative configuration interaction is applied. This can be done in two ways. 

Numerous theoretical studies on DMABN have been carried out, and many of them confirm the greater validity of the TICT model. The main body of such calculations, however, has been limited to the isolated system, while few examples including solvent effects can be quoted. " On the contrary, the phenomenon is strongly related to solvation and thus explicit considerations of solvent interactions are very important to get a more accurate understamding of the experimental evidence on the specific effects due to the presence of polar solvents. Here we summarize the results of the correlated study of DMABN both in vacuo and in solution we have published on the Journal of American Chemical Society. In this study we have used the multireference perturbation configuration interaction (Cl) method, known with the CIPSI acronym, which has been coupled to the PCM-IEF solvation continuum model. ... 

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). 

It is also worth mentioning an alternative approach [77] to describe the localized character of njr excitations. It based on the PCILO (Perturbative Configuration Interaction using Localized Orbitals) method where Cl calculations are carried out in a basis of locally excited determinants. Accordingly, the local structure of nn — excitations is detected by the weights of the appropriate determinant. At the same time, our approach includes no localization procedure for MOs, and excited state local properties arise directly from the main indices (14.35), (14.37), and (14.38). Moreover, the PCILO method is somewhat outdated at least its extension to the DPT methodology seems not reasonable. We cite work [77] as the first paper where the localization phenomena otnn — transitions were discussed from a quantum-chemical viewpoint. 

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