Intermolecular overlap

The exchange repulsion energy in EFP2 is derived as an expansion in the intermolecular overlap. When this overlap expansion is expressed in terms of frozen LMOs on each fragment, the expansion can reliably be truncated at the quadratic term [44], This term does require that each EFP carries a basis set, and the smallest recommended basis set is 6-31-1— -G(d,p) [45] for acceptable results. Since the basis set is used only to calculate overlap integrals, the computation is very fast and quite large basis sets are realistic. 

In crystal structures in which there is appreciable intermolecular overlap of the C-2=C-3 double bonds, irradiation of the solid leads only to (2 + 2) cycloaddition. This is true of reactants 113, which yield in all cases file centro-symmetric dimers 114, even when the reactant conformation is suited also to intramolecular hydrogen abstraction. 

The results of energy partitioning in Li+... OH2 obtained with a number of different basis sets are listed in Table 3. Since intermolecular overlap is small in these kind of complexes (Table 1), we expect the electrostatic model to be a good approximation for classical contributions to the total energy of interaction. Indeed, ZlE cou is to a good approximation proportional to the dipole moment of the water molecule calculated with the same basis set. This can be seen even more clearly in Table 4 where zIEcou is compared with ion-dipole and ion-quadrupole energies obtained from the classical expression of the multipole expansion series 45,95-97) ... 

The determination of the perfect lattice band structure is relatively straightforward. The hole and electron occupy states based respectively upon the highest filled and lowest unfilled molecular orbitals of the parent molecule. The energy levels of these states are broadened into bands by the intermolecular overlap of the molecular orbitals. Knowing the crystal structure and assuming reasonable forms of these orbitals, the band structure may be calculated (Chojnacki, 1968 Le Blanc, 1961, 1962a, b). 

Fig. 14.35 Representative 5,6,11,12-tetrachalcogenotetracenes (top), showing the solid-state ordering and intermolecular overlap for the sulfur derivative 46 (center) and the tellurium derivative 47 (bottom).

Cornelissen, J. R, Romarede, B., Spek, A. L., Reefman, D., Haasnoot, J. G. and Reedijk, J. (1993). Two phases of [Me4N][Ni(dmise)2]2 synthesis, crystal structures, electrical conductivities and intermolecular overlap calculations of a and /3-tetramethylammonium bis[bis(2-selenoxo-l,3-dithiole-4,5-dithiolato)nickelate], the first conductors based on the M(C3S4Se) system. Inorg. Chem., 32, 3720-6. [191]... 

Figure 2 Room-temperature crystal structure of TTF TCNQ [188]. The upper diagram is projection along [ 100], with the unit cell axes a vertical and c horizontal, which shows the stacks of TTF (open circles for the atom positions) and TCNQ (filled circles for atom positions). The lower diagram shows the intermolecular overlap, projected normal to the least-squares molecular planes, along the TTF stack (left), and the TCNQ stack...

FIGURE 13. Properties of neighboring thymine rings into the basic ring in the case of (a) intramolecular overlap in 45a, (b) intermolecular overlap in 45a. 

This observation paves the way to the work by Tsiper and Soos that proposed a mean-field approximation for the calculation of the linear polarizability of molecular crystals and films [25, 62, 63, 64]. The approach is based again on the neglect of intermolecular overlap. A quantum chemical model is adopted for each molecular... 

In this contribution we discuss mm based on pp chromophores, a very interesting class of molecules for applications in molecular photonics and electronics. Push-pull chromophores are both polar and polarizable and this makes the role of intermolecular interactions particularly important. The toy model we propose for clusters of pp chromophores neglects intermolecular overlap, just accounting for classical electrostatic intermolecular interactions, and describes each pp chromophore based on a two state model. The two-state model for pp chromophores has been discussed and validated via an extensive comparison with the spectroscopic properties of several dyes in solution [74, 75, 90], The emerging picture is safe and led to the definition of a reliable set of molecular parameters for selected dyes. This analysis then offers valuable information to be inserted into models for clusters of interacting chromophores, in a the bottom-up modeling strategy that was nicely exemplified in Ref. [90]. 

The reliability of mf approximation for the calculation of static linear and nonlinear susceptibilities is another important result that is expected to hold for mm with negligible intermolecular overlap and not too near to charge instabilities (i.e. to phase transitions or to precursor of phase transitions in finite size systems). This is related to the uncorrelated nature of the gs in these materials, that is fairly well captured within mf. On the opposite, EM does not properly account for tlie molecular polarizability it can possibly describe low-lying excited states in clusters of weakly interacting and hardly polarizable molecules, but it is for sure inadequate to calculate linear and non-linear susceptibilities for mm of interest for applications. 

Painelli and Terenziani discuss the cooperative and collective behavior resulting from classical electrostatic intermolecular interactions in molecular materials with negligible intermolecular overlap. The simple model they employ for clusters of push-pull chromophores neglects intermolecular overlap and describes them using a two-state model. They comment on the excitonic approximation which is expected... 

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