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Packing energies minimizations

Experimental tilt angles have usually an accuracy of at best 3°, leading to an error of about 0.1 A in cell axes. The calculated third cell axis will show a higher deviation. If possible an internal standard should be used for calibration purposes but a higher accuracy will be obtained with a Pawley fit (e.g. fit forP CuPc in Fig. 6) from x-ray powder diffraction data [11]. Especially for packing energy minimization used for simulation methods it is essential to determine the cell parameters as precise as possible. In the case of polymorphism, it is essential to use x-ray powder diffraction to ensure that bulk and investigated nano crystals represent the same modifications. [Pg.414]

Figure 6. Application of packing energy minimization method to poly(ethylene oxide) (16). (a) Starting model of uniform helix (b) stable crystal structure model obtained by energy minimization calculations and (c) the structure determined by x-ray analysis. Figure 6. Application of packing energy minimization method to poly(ethylene oxide) (16). (a) Starting model of uniform helix (b) stable crystal structure model obtained by energy minimization calculations and (c) the structure determined by x-ray analysis.
In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. [Pg.334]

We conclude that a) all degrees of freedom should be unconstrained (including those related to the shape of the sugar rings)/ b) the packing forces can significantly influence the conformation even of a polymer main chain and c) a suitable potential set (with all atoms), used in total energy minimization, can account for these effects. With these considerations in mind, we attempted to model the crystal structures of DeS and an isolated chain of Hep. [Pg.335]

The sophistication of fiber diffraction has grown along with the development of digital computers. These techniques started with the calculation of diffraction intensities for a few proposed models for comparison with the diffraction pattern. At present, parameters of the models can be varied to produce the minimal variance for the observed and calculated diffraction intensities and simultaneously the minimal stereochemical or packing energy. [Pg.523]

As molecular packing calculations involve just simple lattice energy minimizations another set of tests have focused on the finite temperature effects. For this purpose, Sorescu et al. [112] have performed isothermal-isobaric Monte Carlo and molecular dynamics simulations in the temperature range 4.2-325 K, at ambient pressure. It was found that the calculated crystal structures at 300 K were in outstanding agreement with experiment within 2% for lattice dimensions and almost no rotational and translational disorder of the molecules in the unit cell. Moreover, the space group symmetry was maintained throughout the simulations. Finally, the calculated expansion coefficients were determined to be in reasonable accord with experiment. [Pg.152]

Gibson, K. D. and Scheraga, H. A. (1995). Crystal packing without symmetry constraints. 2. Possible crystal packings of benzene obtained by energy minimization from multiple starts. J. Phys. Chem., 99, 3765-73. [185]... [Pg.341]

Fig. 16 Proposed model of molecular packing in the crystalline phase of PF8 energy-minimized structure (backbones in red) as viewed along a the c axis and b the b axis c an expanded view with side chains hidden to demonstrate twisting of the backbone. A few selected pairs of M-octyl side chains are highlighted in green in (a) and (b) to allow for easier identification. Reprinted with permission from [73]. (2004) by the American Chemical Society... Fig. 16 Proposed model of molecular packing in the crystalline phase of PF8 energy-minimized structure (backbones in red) as viewed along a the c axis and b the b axis c an expanded view with side chains hidden to demonstrate twisting of the backbone. A few selected pairs of M-octyl side chains are highlighted in green in (a) and (b) to allow for easier identification. Reprinted with permission from [73]. (2004) by the American Chemical Society...
In Section II we define energy landscapes, and we present the formahsm that relates potential energy minima to the thermal properties of supercooled liquids and glasses. Section III discusses the characterization of voids in dense particle packings, and how this approach, combined with energy minimization techniques, can yield powerful new insights into the mechanical... [Pg.36]


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