BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 123

experiment. We may test a theory by conducting a simulation using the same
model. We may test the model by comparing with experimental results. We may also
carry out simulations on the computer that are difficult or impossible in the labora-
tory (for example, working at extremes of temperature or pressure). Ultimately
we may want to make direct comparisons with experimental measurements made
on species materials, in which case a good model of molecular interactions is es-
sential. The aim of so-called ab initio molecular dynamics is to reduce the amount
of tting and guesswork in this process to a minimum. On the other hand, we may
be interested in phenomena of a rather generic nature, or we may simply want to
discriminate between good and bad theories. When it comes to aims of this kind,
it is not necessary to have a perfectly realistic molecular model; one that contains
the essential physics may be quite suitable [122, 132].

1.3.2.1 MILESTONES IN MD SIMULATIONS OF SWNT FORMATION
As we revisit the milestones in MD simulations of SWNT nucleation and growth,
the degree to which each investigation satisfies each of these four hypothetical
“tasks” will be emphasized [129].

1.3.2.1.1 REBO-BASED MD SIMULATIONS

The Ters off potential and its application to hydrocarbon systems, called the
REBO potential, describes bond breaking/formation using a distance-dependent
many-body bond order term and correctly dissociating attractive/repulsive di-
atomic potentials. This approach is nevertheless based on a local molecular me-
chanics approach, and therefore lacks explicit descriptions of quantum phenom-
ena and Coulomb/vander Waals interactions. Hence, changes in the electronic
structure of a dynamic, molecular system are not described. In particular, neither
the evolution of conjugation and aromatic stabilization of carbons (important for
sp 2 -hybridized carbon), nor charge transfer effects and the near degeneracy of
metal d-orbitals (important for the catalyst) can be captured by MD simulations
based on the Ters off and REBO potentials. Besides these grave decencies, the
standard REBO potential has several other well-known problems. For example,
in modeling gas-phase carbon densities it overestimates the sp 3 fraction compared
to corresponding DFT simulations, while it underestimates the sp fraction which
is of paramount importance with respect to fullerene cage self-assembly [129].

1.3.2.1.2 CPMD SIMULATIONS

CPMD simulations have been employed by two research groups for simulating
the growth process of metal-catalyzed SWNT growth. Remarkably, the study by
researchers, was performed even before the first REBO-based MD simulations by
the Maruyama group. As stated above, CPMD is a more accurate MD approach
compared to REBO based MD, since it includes changes in the electronic structure