Environmental Engineering Reference
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1.3.2.1.5
DFTB-BASED MD SIMULATIONS
Neither REBO-based nor first principle-based MD simulations have succeeded
in modeling the nucleation and sustained growth of a clean hexagononly SWNT
from scratch as routinely achieved experimentally. This is despite the use of such
tricks as injecting carbon atoms into the middle of the metal catalyst particle. It
seems that one should “bite the bullet” and deal somehow with the complicated
electronic structure of the evolving sp 2 -network and transition metal clusters at
lower computational cost. The major obstacle for such a “cheap” CPMD ap-
proach is the limited availability of metal parameters in conventional semi em-
pirical quantum chemical methods. Most notable in this respect are the MNDO/d
or PM6 codes. However, the metal parameters are typically designed for single
metal atom systems and are not suitable for the treatment of metal nanoparticles.
The DFTB/MD approach, which is a quantum mechanics/molecular dynamics
(QM/MD) technique based on the DFTB electronic structure method, can play a
role to in a gap between classical and the principle MD simulations. The DFTB
method is approximately two orders of magnitude faster than first principles DFT
and therefore enables longer simulations and provides more adequate model sys-
tems for non-equilibrium dynamics of nanosized clusters with quantum mechani-
cal treatment of electrons. In addition, metal-carbon parameters for DFTB have
recently been developed by the Morokuma group. A nite electronic temperature
approach ensures the applicability of the DFTB/MD method for nanometer size
metal particles with high electronic densities of states around the Fermi level, as
it allows the occupancy of each molecular orbital to change smoothly from 2 to 0
depending on its orbital energy. This approach effectively incorporates the open
shell nature of the system due to near-degeneracy of iron d-orbitals, as well as
carbon dangling bonds. We have found that in the absence of electronic tempera-
ture the iron cluster is much less reactive. This is considered to be an artifact of
the simulation of a near-closed shell electronic wave function in that case Initially,
we employed a (5,5) armchair SWNT fragment attached to an Fe 38 cluster as a
model system. The open end of the SWNT seed was terminated by hydrogen
atoms, whereas the other end was bound to surface iron atoms of the Fe 38 cluster
[129, 144].
1.3.2.1.6 SIMULATION OF ADSORBED MONOLAYERS IN SMOOTH
SUBSTRATES
The study of two-dimensional systems of adsorbed atoms has attracted great at-
tention because of the entire question of the nature of two-dimensional melting. In
the absence of a periodic substrate potential, the system is free to form an ordered
structure determined solely by the inter particle interactions. As the temperature
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