18 research outputs found
The confined hydrogen atom with a moving nucleus
We study the hydrogen atom confined to a spherical box with impenetrable
walls but, unlike earlier pedagogical articles on the subject, we assume that
the nucleus also moves. We obtain the ground-state energy approximately by
means of first--order perturbation theory and by a more accurate variational
approach. We show that it is greater than the one for the case in which the
nucleus is clamped at the center of the box. Present approach resembles the
well-known treatment of the helium atom with clamped nucleus
Systematic first-principles study of impurity hybridization in NiAl
We have performed a systematic first-principles computational study of the
effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus,
and sulfur) on the orbital hybridization and bonding properties in the
intermetallic alloy NiAl using a full-potential linear muffin-tin orbital
method. The matrix elements in momentum space were used to calculate real-space
properties: onsite parameters, partial densities of states, and local charges.
In impurity atoms that are empirically known to be embrittler (N and O) we
found that the 2s orbital is bound to the impurity and therefore does not
participate in the covalent bonding. In contrast, the corresponding 2s orbital
is found to be delocalized in the cohesion enhancers (B and C). Each of these
impurity atoms is found to acquire a net negative local charge in NiAl
irrespective of whether they sit in the Ni or Al site. The embrittler therefore
reduces the total number of electrons available for covalent bonding by
removing some of the electrons from the neighboring Ni or Al atoms and
localizing them at the impurity site. We show that these correlations also hold
for silicon, phosporus, and sulfur.Comment: Revtex, 8 pages, 7 eps figures, to appear in Phys. Rev.
Beam Orientation Optimization for Intensity Modulated Radiation Therapy using Adaptive l1 Minimization
Beam orientation optimization (BOO) is a key component in the process of IMRT
treatment planning. It determines to what degree one can achieve a good
treatment plan quality in the subsequent plan optimization process. In this
paper, we have developed a BOO algorithm via adaptive l_1 minimization.
Specifically, we introduce a sparsity energy function term into our model which
contains weighting factors for each beam angle adaptively adjusted during the
optimization process. Such an energy term favors small number of beam angles.
By optimizing a total energy function containing a dosimetric term and the
sparsity term, we are able to identify the unimportant beam angles and
gradually remove them without largely sacrificing the dosimetric objective. In
one typical prostate case, the convergence property of our algorithm, as well
as the how the beam angles are selected during the optimization process, is
demonstrated. Fluence map optimization (FMO) is then performed based on the
optimized beam angles. The resulted plan quality is presented and found to be
better than that obtained from unoptimized (equiangular) beam orientations. We
have further systematically validated our algorithm in the contexts of 5-9
coplanar beams for 5 prostate cases and 1 head and neck case. For each case,
the final FMO objective function value is used to compare the optimized beam
orientations and the equiangular ones. It is found that, our BOO algorithm can
lead to beam configurations which attain lower FMO objective function values
than corresponding equiangular cases, indicating the effectiveness of our BOO
algorithm.Comment: 19 pages, 2 tables, and 5 figure