Quantum-effective pair potential structures

In general, in the study of systems at equihbrium the main advantage of the semiclassical approaches is that they can be utilized in simulation work by following essentially the same techniques as in the classical case (i.e., one works with Ng particles). Moreover, whenever they can be applied, semiclassical approaches can work excellently and represent a substantial saving in computational effort as compared to the exact PI xP-calculations. On the negative side, even the best semiclassical approaches cannot cope with very large quantum diffraction effects. Actually, they do not capture all the information needed to characterize the quantum system (e.g., some g (r) structures cannot be computed). In the literature of this field there are different names for these pair potentials (e.g., classical effective potentials, semiclassical effective potentials, quantum effective pair potentials, etc.). The absence of external fields will be assumed in this section, and the system will be ruled by the Hamiltonian... 

This basic semiclassical approach can be found fully explained in the book by Feynman [13], and only a number of key facts will be given here. It is important to realize that this approach contains much more information than just the quantum effective pair potential. As shown by the present author, the GFH gives a whole picture for quantum particles in a many-body systan [35], and some of its results can be onployed under a wide range of conditions to complement exact PI treatments, as will be seen later in connection with the structure studies (Section IV). 

Strictly spealmg, the SVP potentials provide approximations to the instantaneous correlations ET2 in r-space [116]. However, owing to the Ganssian smearing contained in the convolutions, the g (r) obtained are always expected to be much closer to the actual continuous linear response PI-LR2 correlations than to the actual PI-ET2 correlations. This is a behavior that will become more pronounced with increasing quantum effects [116], Accordingly, with the use of the SVP radial structure the conventional formula Eq. (66) can be utilized to estimate the instantaneous structure factor The coherent (pair) part of the latter... 

Almost all of the calculations on polyads were performed for isolated base complexes thus neglecting effects of nucleic backbone, solvent and also of entropic contributions. Erqreriments most directly related to quantum-chemical results are gas phase investigations on base pairs.Nevertheless, it makes also sense to compare the results of quantum-chemical studies on base pairs or polyads to complete three-dimensional nucleic acid structures. One should realize, however, that there is a long way to go from results of quantum-chemical studies on building blocks to nucleic acid structure and function. The power of this approach is that it can help to separate the effects of different parts of the nucleic acid, such as backbone and bases, on the overall structure. In addition, it provides useful information on charge distribution and electrostatic potentials relevant for the determination of interaction sites with cations such as K, the most abundant cation in cells. Finally, the interaction energies can be studied in great detail. For example non-additive contributions can be determined. 

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