Fragments, defined

Figure 12. In order to check the experimental Patterson map for a known structural fragment it must be converted into Patterson space first. This is shown in the above sketch for the fragment of a body centered cube which is projected along the 4-fold axis (a). The corresponding motif in Patterson space is shown in (b). Note, that the atom in the center of the fragment defines a 2-fold axis what reduces the number of different peaks in the Patterson map.

Each resolution implies its own case of the intra-system equilibrium. For example, the L-resolution corresponds to a totally constrained ( frozen ) electron distribution, with all local, infinitesimal volume elements considered as being mutually closed, while the g-resolution represents the opposite extreme of a totally relaxed electron distribution, with all local volume elements regarded as mutually open. Similarly, the levels of intermediate resolutions apply to cases of partially constrained equilibrium, in which all fragments defining the partitioning of a molecular system M ... 

The Minimum Topological Difference (MTD) approach developed by Z. Simon and T. Oprea, especially in its newer MTD-PLS variant employing the PLS regression analysis, successfully relates the activity to the presence and physicochemical parameters of the atoms and fragments defined over a quasi-3D hypermolecule as well as global physicochemical descriptors. Despite some methodological problems and limitations, this approach is promising and in active current development. ... 

Conversely, reaction of linear 4 -transDPyP imits with 90°-angular connectors, such as ds-[PtCl2(PhCN)2] [49], [ReX(CO)s] [55,57], and [M(dppp) (OXf)2] (M = Pd, Pt) [52,53], yielded the corresponding 4 + 4 metaUacycles in which the metal fragments define the corners and the porphyrins the sides of the molecular square, and are actuaUy oriented (as an average) perpendicularly to its plane (Fig. 9). 

It is important to stress that only an approximate W wave function can be obtained if the partition of the electrons among different groups is kept fixed in all the configuration functions This approximation can be called the simplified group function (SGF) method [45], in contrast to the full method where all the possible partitions of the N electrons are allowed. Nevertheless the SGF approximation is quite good in all the cases where there are no important covalent interactions. This is the case for most of the van der Waals complexes and also for interacting two-electron bonds [81], or for fragments defined as an ensemble of localized bonds. 

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