Direct molecular dynamics expansion

In order to interpret the results of our experiments, optimal-control calculations were performed where a GA controlled 40 independent degrees of freedom in the laser pulses that were used in a molecular dynamics simulation of the laser-cluster interactions for Xejv clusters with sizes ranging from 108 to 5056 atoms/cluster. These calculations, which are reported in detail elsewhere [67], showed optimization of the laser-cluster interactions by a sequence of as many as three laser pulses. Detailed inspection of the simulations revealed that the first pulse in this sequence initiates the cluster ionization and starts the expansion of the cluster, while the second and third pulse optimize two mechanisms that are directly related to the behaviour of the electrons in the cluster. We consistently observe that the second pulse in the three-pulse sequence arrives a time delay where the conditions for enhanced ionization are met. In other words, the second pulse arrives at a time where the ionization of atoms is assisted by the proximity of surrounding ions. The third peak is consistently observed at a delay where the collective oscillation of the quasi-free electrons in the cluster is 7t/2 out of phase with respect to the driving laser field. For a driven and damped oscillator this phase-delay represents an optimum for the energy transfer from the driving force to the oscillator. 

Close examination of the radii of ration from the molecular dynamirs simulations reveals that the chains are not completely ideal Overall the chains exhibit nearly ideal scaling behavior for whidi R oc Locally, however, the chains are found to be expanded relative to a freely jointed chain of the same length. This local expansion is a result of local intramolecular excluded volume which has not been completdy screened out in the melt. Thus in order to predict accurately the intermolecular structure one needs to correct for local deviations from Flory s ideality hypothesis. We were able to make this correction by employing the ra(k) directly computed from the molecular dynamics simulation. When the PRISM theory is then used to calculate g(r) from the actual, simulated d>(kX excellent agreement is seen in Fig. 3 between the theory and the simulation... 

Macroscopic models have been applied to the MALDI plume, including a hydrodynamic approach and that of a pre-accelerated adiabatic expansion. These cannot directly account for mixed gas/clusters in the ablation regime, but are still useful as a first approximation. They also cannot account for changes in plume development as the desorption/ablation crater shape changes.In contrast, the mixed-phase aspect of ablation is a natural part of molecular dynamics, even though it is not computationally possible to include the full temporal and spatial extent of a real experiment. Nevertheless, molecular dynamics has illuminated numerous aspects of the phase transition aspect of MALDI. " - ... 

Much of the published work on extrusion has attempted to correlate process conditions and formulation with final product properties. Correlations are almost always found, and may be systematic within the particular set of variables examined, but do not survive when further parameters are examined in other studies. This makes them of limited use in even qualitative explanation of product properties. Re-examination of published data, bearing in mind the effect of material states on the expansion process after the die, shows that there is a systematic explanation that can be related to the material properties of the extruded mass, during the dynamic formation of bubbles and cellular structures. However, because of the transformation of materials within the barrel, both at the microscopic and molecular level, it is unrealistic to expect test methods on raw materials alone to relate directly to product structures and... 

For all but the very smallest systems, (such as HeH and even there it is very expensive), it is not possible in practice to calculate the full potential surface, with a grid fine enough that it can be directly used for solving the (nuclear) dynamical problem in Van der Waals molecules (or for scattering calculations). Moreover, such a numerical potential would not be convenient for most purposes. Therefore, one usually represents the potential by some analytical form, for instance, a truncated spherical expansion (1) or another type of model potential (cf. sect. 2). The parameters in this model potential can be obtained by fitting the ab initio results for a limited set of intermolecular distances and molecular orientations. Since we have encountered some difficulties in this fitting procedure which we expect to be typical, we shall describe our experience with the ( 2114)2 and (N2)2 cases in some detail. At the same time, we use the opportunity to make a few comments about the convergence of the spherical expansion used for (Njjj and about the validity of the atom-atom model potential applied to both ( 2114)2 and (Njjx. 

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