High-temperature quantum chemical molecular dynamic

Chapter 31 - High-temperature quantum chemical molecular dynamics simulations of carbon nanostructure self-assembly processes. Pages 875-889, Stephan Irle, Guishan Zheng, Marcus Elstnerand Keiji Morokuma... 

High-temperature quantum chemical molecular dynamics simulations of carbon nanostructure self-assembly processes... 

Theoretical approaches, developed over the past 40 years of quantum chemistry, have recently become very helpful tools for developing an atomic-level understanding of the processes involved in high-temperature carbon chemistry. Interestingly, a combination of two theoretical approaches developed at opposite ends of this time-scale has proven to be extremely fruitful for such studies, namely the relatively new quantum chemical molecular dynamics (QM/MD) approach [14], using improved versions of early-day Extended Hiickel electronic structure method [15-17] for the calculation of potential... 

When addressing problems in computational chemistry, the choice of computational scheme depends on the applicability of the method (i.e. the types of atoms and/or molecules, and the type of property, that can be treated satisfactorily) and the size of the system to be investigated. In biochemical applications the method of choice - if we are interested in the dynamics and effects of temperature on an entire protein with, say, 10,000 atoms - will be to run a classical molecular dynamics (MD) simulation. The key problem then becomes that of choosing a relevant force field in which the different atomic interactions are described. If, on the other hand, we are interested in electronic and/or spectroscopic properties or explicit bond breaking and bond formation in an enzymatic active site, we must resort to a quantum chemical methodology in which electrons are treated explicitly. These phenomena are usually highly localized, and thus only involve a small number of chemical groups compared with the complete macromolecule. 

HMX (1,3,5, 7-tetranitro-l, 3,5,7-tetraazacyclooctane) is widely used as an ingredient in various explosives and propellants. A molecular solid at standard state, it has four known pol5miorphs, one of which, the 8 phase is comprised of six molecules per unit cell, as depicted in Fig. 10. We study the chemical decomposition of the dense fluid of this phase by conducting a high-density and temperature (p = 1.9 g/cm, T = 3500 K) quantum mechanical based molecular dynamics simulation. [50] To our knowledge, this is the first reported ab initio based/molecular dynamics study of an explosive material at extreme conditions for extended reaction times of up to 55 picoseconds, thus allowing the formation of stable product molecules. 

This confusion does not avoid that, for the essential problems of structural chemistry, it remains relatively simple for us to avoid the dynamic problems (the models for all sciences since the eighteenth century) and base the entire structure of chemical systems essentially in the steady state solutions of the Schrodinger equation. This is a good enough approximation since the spacing between electronic levels is sufficiently high for, at the temperatures common at the Earth s surface, we only have the fundamental electronic levels filled (Bent, 1965). In fact, apart from the explanatory discourse between levels 1 and 2 (the structural or quan-tum/electronic level and the reactional or statistical/molecular level), particularly for the description of transition states, photochemical reactions, the ground quantum level will be sufficient for the more intricate thematic descriptions. 

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