Relaxation time reaction limitation with comparable

Molecular dynamics is a true first principles dynamic molecular model. It simply solves the equations of motion. Given an intermolecular potential, MD provides the exact spatial and temporal evolution of the system. The stiffness caused by fast vibrations compared with slow molecular relaxations demands relatively small time steps and challenges current simulations. As an example, the time scale associated with vibrations is a fraction of a picosecond, whereas those associated with diffusion or reaction may easily be in the seconds to hours range depending on the activation energy. Consequently, MD on a single processor is usually limited to short time and length scales (e.g., pico- to nanoseconds and 1-2 nm). 

The usefulness of computational methods would of course be quite limited if environmental effects could not be taken into proper account, since almost all of the above-mentioned processes occur in solution. As a consequence, even a qualitative agreement with experiments requires the use of a suitable solvation model. The inclusion of environmental effects involves additional difficulties Not only should the solvation model be able to provide an accuracy comparable to that attained in vacuo, but in solution any problem involving excited states becomes intrinsically dynamic [41]. The solvent reaction field couples the ground-state density with the density correction and the orbital relaxation arising from the electronic transition. Furthermore, the coupling is modulated by the solvent relaxation times [41]. 

If there is a fundamental kinetic limitation at the standard process temperature, it may be possible to adopt more severe process conditions, bearing in mind that intensified equipment will entail much shorter residence times. Thus the balance between the desired and undesired reactions may be influenced favourably. Certain processes may spuriously appear to be kinetically limited, especially if the reaction involves intermediates which are only present in small concentrations, e.g. dissolved oxygen in fermentations or organic oxidations. The key factor is the half-life of the relevant reacting species. If this is short, compared with the mixing rate or circulation time in the reactor, then the system is limited by the prevailing fluid dynamics rather than the kinetics. Obviously, when the kinetics are not limiting, the fluid environment must be intensified to relax the other restrictions. 

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