First Principles Simulations Models

While ab initio molecular dynamics simulations of condensed phase system hold great promise for accurate modeling of condensed phase processes, we anticipate that their use in large-scale simulations of reactions of energetic materials will not be feasible for several years. Therefore, until the computational limitations are eased, then molecular dynamics simulations of energetic materials in the condensed phase will be restricted to classical descriptions of reactions. 

Astronautics and Aeronautics, P. Zarchan, Editor in Chief (American Institute of Aeronautics and Astronautics, Reston, VA, 2000, pp. 501-531. 

5- dinitrobenzene (DEMSOD). (49) 1,2,3-Propanetriol trinitrate, beta modification (CORYIR).(50) Pentaerythritol tetranitrate (PETN), form I (PERYTN10). (51) Pentaerythritol tetranitrate, form II (PERYTN01). 

George. AD-C048 931 (92-0134), p. 155, Apr. 91, CPIA Abstract. No. 92, 0149, AD D604 542, C-D, Chemical Propulsion Information Agency, 10630 Little Patuxent Parkway, Suite 202, Columbia, MD 21044-3200. 

Energetic Materials, Part l Decomposition, Crystal and Molecular Properties Theoretical and Computational Chemistry, Vol. 12 2003 Elsevier B.V. All rights reserved. 

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The following sections highlight some important considerations regarding the most critical decisions related to the spray drying process, viz. selection of scale, atomizer, and key process parameters. Common challenges associated with the operation of the process will also be addressed. At the end of the section, a scale-up methodology based on scientific first principles, simulation models, and process characterization techniques are presented. 

The nature of the methanol-zeolite interaction has been shown to be sensitive to a number of parameters and as such has proved to be a good benchmark for judging the reliability of quantum chemical methods. Not only are there a number of possible modes whereby one and two molecules interact with an acidic site (245), the barrier to proton transfer is small and sensitive to calculation details. Recent first-principles simulations (236-238) suggest that the nature of adsorbed methanol may be sensitive to the topology of the zeolite pore. The activation and reaction of methane, ethane, and isobutane have been characterized by using reliable methods and models, and realistic activation energies for catalytic reactions have been obtained. 

The catalytic behavior can further be changed by cluster size and impurity doping. To understand the origin of this size-dependent and element-specific behavior, the atomic structure and the electronic spectra of these model catalysts were studied by extensive first-principles simulations, and the optimized structures of Aug (with two relevant isomers), AU4, and AugSr adsorbed on MgO(FC) are shown in Figure 5, before (a-d) and after (e-h) O2 adsorption. 

Point Defects and Diffusion by C. P. Flynn, Clarendon Press, Oxford England, 1972. A definitive treatise on the subject of point defects and diffusion as seen from the perspective which predates the routine use of first-principles simulations to inform models of diffusion. 

A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

Babin, V, Medders, G. R., and Paesani, F. (2012). Toward a universal water model First principles simulations from the dimer to the liquid phase, J. Phys. Chem. Lett. 3, 24, pp. 3765-3769, doi 10.1021/jz3017733, URL http //pubs.acs.org/doi/abs/10.1021/jz3017733. 

Nowadays, several process simulators such as Aspen Plus and Aspen HYSYS are commercially available for simulating complete chemical processes. Common process units and a property database for numerous chemicals are available in such simulators. However, models for less common and/or new process units (for example, membrane separation) are not readily available in the simulators, but they may be available in the literature or can be developed from first principles. Mathematical model for a new process unit can be implemented in Aspen Custom Modeler (ACM), and then it can be exported to (included in) Aspen Plus or Aspen HYSYS for simulating processes having a new process unit besides common process units such as heat exchangers, compressors, reactors and columns. Process simulators for simulation and ACM for implementing models of new process units are... 

A mechanistic first principle-based model to study the coating variabihty in a pan coater is being developed. The maximum coating efficiency with minimal variability was accomplished at the optimal coating condition of 32° vessel tilt and rotational speed of 30rpm, in both experiments and simulations. The coating variability decreased with time under all conditions. 

If computational power permits, first-principles molecular modeling such as ab initio MD can provide the most accurate information without adjustable parameters. However, even the most powerful computer system available today is limited to first-principles solution of molecular information with a certain size. As a result, the hybrid methods such as QM/MM or ONIOM (where different levels of theory for one calculation are involved) are often used (Fig. 1). As an example of future prospects, molecular design and characterization of nanocomposites using computer simulation are also briefly mentioned. Atomistic modeling of clay nanocomposites is a very promising field in clay mineral materials science. 

Reactive molecular dynamics (ReaxMD) is based on the Reax force field (ReaxFF) parameterized by fitting to a training set of QM data. Compared to first-principles simulations, with the ReaxFF model it is possible to speed up the calculation by several orders of magnitude. However, due to the enormous complexity of the underlying mathematical expressions, ReaxFF is -10 to 100 times more expensive computationally than simple (nonreactive) FFs. 

One of the major uses of molecular simulation is to provide useful theoretical interpretation of experimental data. Before the advent of simulation this had to be done by directly comparing experiment with analytical (mathematical) models. The analytical approach has the advantage of simplicity, in that the models are derived from first principles with only a few, if any, adjustable parameters. However, the chemical complexity of biological systems often precludes the direct application of meaningful analytical models or leads to the situation where more than one model can be invoked to explain the same experimental data. 

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