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Research Area Summary:
We predict how molecules and clusters respond when subjected to
mechanical, electromagnetic, or chemical probes. Our interest is in
exploiting properties which allow for information storage,
environmental sensing, or energy conversion. This work is accomplished
by predicting stabilities, reactivities, geometries, electronic and
vibrational spectra, and magnetic properties.
 | FIG. 1.
The Mn12 molecular magnet (click for a larger picture).
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Molecular magnets:
Molecular magnets are of fundamental interest because they hold their
magnetic reorientation at relatively high temperatures and because
they exhibit the phenomenon of resonant tunneling of
magnetization. These systems are typically composed of 4-15 transition
metal atoms which are held in place by organic ligands, leading to
sizes in the range of 50-200 atoms. Pictured here is the
Mn12-Acetate molecular magnet, which has become the canonical
prototype. Other molecular magnets studied recently by our group can
be seen here. One
of our interests in this area is to predict both the reorientation
temperatures and resonant tunneling fields( which primarily depend on
the spin-orbit interaction) and to determine how to control
computationally these features. A longer range goal is to determine
the environmental conditions for which such systems can be used in
device applications.
In order to predict how the energy barriers develop we perform density-functional
calculations and then determine how the spin-orbit-energy depends on the chosen
axis of quantization. Our work shows that density-functional theory leads to
very high accuracy for predicting second-order magnetic anisotropy energies.
The prediction of higher-order magnetic anisotropies has been a challenge due
to the small energy scales involved and because many different interactions
contribute to the higher-order magnetic anisotropies. We have recently suggested that a
vibrationally induced modification of the spin-orbit interaction may be an
important interaction. Our calculations on fourth-order anisotropies in
the Mn12-Acetate molecule are in very good agreement with experiment.
Point of contact:
Mark.Pederson@nrl.navy.mil (Privacy Advisory)
 | FIG. 2.
Partially ionic N5/N3 dimer (click for a larger picture).
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Energetic materials:
The DoD and civilian industries are interested in developing a new
family of high density energetic molecules (HDEM). To determine if a
molecule is a good candidate for energy storage it is necessary to
find geometrical arrangements of atoms that are locally stable but
energetically unstable with respect to the ground state of the
system. Locally stable geometries may be confirmed by calculating the
vibrational energies of the system and determining whether all the
frequencies are real. A perfect cube of eight nitrogens,
octaazacubane, has been predicted to be vibrationally stable but paths
toward synthesis have not been found. Recently a cationic nitrogen
pentamer has been determined to be stable. In the work illustrated
here we determined that a partially ionic dimer composed of a
positive pentamer and a negative trimer would be vibrationally
stable. Possibilities for using this configuration as a route toward
synthesis of octaaazocubane are unlikely since our calculations also
show that a reasonably small energy barrier allows for decomposition
into molecular nitrogen. We have further shown that the the N5-N3
intermediate may also tranform into azidopentazole which, while
putative, is computationally the most stable of all known N8
conformers. Another picture of a different energetic molecule studied
by our group can be seen here.
Point of contact:
Mark.Pederson@nrl.navy.mil (Privacy Advisory)
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