Catalysis of the CCl2F2 - Dis mutation Reaction on AlF3.
HFCs are synthesised via halogen exchange reactions of
CFCs and HF. Recently, there has been much interest in
the use of aluminium fluoride (AlF3) as such a catalyst [1].
High surface area (HS) AlF3 has been shown to act as a
very efficient catalyst for these reactions. In some cases
it is even more effective than the widely used Swartz
catalyst based on antimony pentafluoride.
We used state of the art hybrid density functional theory
to investigate the interaction of CCl2F2 with and to
study the energetics of the proposed reaction pathway.
The CRYSTAL program was used to
perform these calculations. Reaction pathways for the
dis mutation reaction were obtained using the nudged
elastic band (NEB) algorithm, of which has recently been
implemented in the CRYSTAL program. This algorithm
requires the structures of the initial and final states of the
transition pathway and an initial guess of the pathway.
The reaction pathway for the catalysis of 2CCl
_{2}F
_{2} + CCl
_{3} on the surface of β-AlF
_{3}
Spin-qubits in Carbon Peapods.
Spin chains have the potential to provide the controlled
interactions needed for quantum computing. Carbon is a
candidate host for spin qubits because in 12C materials the
small spin-orbit coupling and absence of hyperfine coupling
ensures long spin coherence times. Carbon peapods, that is,
single-walled carbon nanotubes (SWNT) containing
fullerenes, have been proposed as particularly suitable spin
chain systems. The fabrication of nanoscale electronic
devices, such as field effect transistors, with carbon
peapods containing various endohedral fullerenes is well
established. When spin active metallic atoms such as Sc are
incarcerated in a carbon cage, the system develops
hybridized orbitals resulting in an unpaired electron
delocalized across the fullerene cage potentially a near
ideal qubit.
The CRYSTAL code was used to perform calculations based
on density functional theory (DFT).We find well-defined
spin-1/2 qubits on the fullerenes, with strong evidence for
a nearest-neighbour Heisenberg exchange interaction. In
order to describe the influence on the spin-qubits localized
on the fullerenes of propagating electrons or holes in the
nanotube, it is necessary to go beyond DFT to a model
which is capable of describing the low-energy charge-spin
excitations of the system. We conjecture a generic
Hubbard-Anderson model; which captures the Heisenberg
exchange between spins along the fullerene chain and the
Kondo exchange interaction between localized spins on the
fullerenes and spins of propagating electrons or holes in
the nanotube.
CRYSTAL in Parallel
Massively Parallel CRYSTAL Calculations
The code implements Hartree Fock and density functional theory using a local Gaussian basis set providing for the accurate calculation of energy and forces for structural optimisation and a wide variety of material properties.
The massively parallel implementation of CRYSTAL allows systems with ~1000 atoms per unit cell to be studied routinely on parallel computers. The two key computational steps are the evaluation of the Hamiltonian through the calculation of matrix element integrals and the solution of the Kohn Sham equations through diagonalisation of the Hamiltonian.