Perturbation configuration interaction using

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). 

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). 

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 Moreover, the PCILO method is somewhat outdated at least its extension to the DPT methodology seems not reasonable.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. 

An alternative CNDO procedure for Cl, developed at the same time as CNDO/S but oriented toward ground-state properties is the PCILO (perturbative configuration interaction using localized orbitals) method of Malrieu, Claverie, Diner, and co-workers. °" Its major strength and weakness lie in the use of intuitively construaed hybrid atomic orbitals as zeroeth-order localized MOs. This leads to conceptually appealing interpretations at the expense of some arbitrariness in the treatment. 

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. 

Ala = alanine Asp = aspartic acid Gly = glycine His = histidine Leu = leucine Met = methionine Om = ornithine PCILO = perturbation configuration interaction using localized orbitals Phe = phenylalanine Tyr = tyrosine Val = valine. 

NOE = nuclear Overhauser effect PCILO = perturbative configuration interaction using localized orbitals pmf = potential of mean force. 

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