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transition state theory. Because traditional kinetic Monte Carlo methods require
predefined reactions and make the lattice approximation, they are not directly
applicable to a complex process such as CNT growth. Nevertheless, alternative
schemes have been proposed for circumventing the lattice approximation by calcu-
lating reaction barriers on the fly [
89
]. The bond order/bond distance relationship
already present in ReaxFF would provide a natural tool for the development of an
automated reaction search procedure, enabling kinetic Monte Carlo simulations
within the ReaxFF framework. Such simulations would be capable of looking at
CNT growth over a significantly longer timescale than ReaxFF RD.
As effective as reaxFF is for handling reactive systems and processes in their
ground-state, it is unable to describe the dynamics of electrons and systems with
excited electronic states. QM-MD is also limited mostly to ground-state dynamics
or to a very small number of excited electronic states (see [
90
] for further discussion
on this). The following section presents our progress in addressing this problem
with a mixed quantum-classical force field method, the eFF.
3.2.2 Non-Adiabatic Excited Electronic State Dynamics with an FF
A significant number of processes involve excited electronic states, whose character -
and representation in a theoretical method - depends strongly on the degree
of excitation involved. Low-level electron excitations of molecules can initiate
radical reactions, isomerize bonds, and induce transfers of electrons. Such pro-
cesses can be studied effectively using conventional QM, using a wavefunction
formed from Hartree-Fock or Kohn-Sham orbitals. At the other extreme, high-
level excitations result in the formation of a weakly-coupled plasma, where bonding
and chemistry vanishes, and electrons act as point particles interacting with nuclei
via classical electrostatics. Such systems can be studied using classical plasma
simulations techniques, i.e., particle-in-cell codes.
However, in between low and high extremes of electron excitation lies a rich
variety of phenomena where the electrons are far removed from the ground state,
yet remain strongly coupled to the nuclei, so that remnants of bonding and chemis-
try persist.
Understanding the properties of warm dense matter present in moderately
excited systems is of crucial importance to developing a range of new technological
enterprises. For example, in inertial confinement fusion, liquid deuterium is com-
pressed by a shock wave, causing molecules to dissociate into atoms, atoms to
ionize into plasma, and metallic conducting phases to form. Knowledge of how
these phases interact could contribute to the design of improved fuel pellets.
Other examples come from the semiconductor industry, where electron beams
are used to etch ultra-fine features (
35 nm) into silicon, the nuclear industry,
where the interior of reactors must be protected from the passage of fast charged
particles, and the biological community, where synchrotron radiation could enable
single molecule X-ray diffraction, if the dynamics of highly excited and ionized
biomolecules could be understood. In the above cases, theory could play a critical
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