4,874 research outputs found
A survey of impulsive trajectories Final report
Literature survey of astrodynamics problems on intercept, transfer, and rendezvous trajectorie
Recent trends in cancer mortality in the UK
Comparisons of survival rates of given diseases with different treatments or in different places often gave misleading results until the introduction of controlled trials. Recent reports of relatively low survival rates following the treatment of cancer in the UK compared to the rates in other countries, not based on controlled trials, may consequently be misleading. Their validity has, therefore, been tested by comparing the levels and trends in mortality – the ultimate criterion by which the success or failure of any system of care can be judged. For this purpose, rates and trends in rates over 20–50 years have been compared in five European countries of similar economic status (France, Italy, the Netherlands, Sweden, and the UK). The UK rates are not generally worse than those in the other countries and are sometimes better. Exceptions were cancer of the lung, large bowel, and breast, the first of which is explained by differences in the prevalence of smoking
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
Phase changes in selected Lennard-Jones X_{13-n}Y_n clusters
Detailed studies of the thermodynamic properties of selected binary
Lennard-Jones clusters of the type X_{13-n}Y_n (where n=1,2,3) are presented.
The total energy, heat capacity and first derivative of the heat capacity as a
function of temperature are calculated by using the classical and path integral
Monte Carlo methods combined with the parallel tempering technique. A
modification in the phase change phenomena from the presence of impurity atoms
and quantum effects is investigated.Comment: 14 pages, 13 figures. submitted to J. Chem. Phy
Local Variational Principle
A generalization of the Gibbs-Bogoliubov-Feynman inequality for spinless
particles is proven and then illustrated for the simple model of a symmetric
double-well quartic potential. The method gives a pointwise lower bound for the
finite-temperature density matrix and it can be systematically improved by the
Trotter composition rule. It is also shown to produce groundstate energies
better than the ones given by the Rayleigh-Ritz principle as applied to the
groundstate eigenfunctions of the reference potentials. Based on this
observation, it is argued that the Local Variational Principle performs better
than the equivalent methods based on the centroid path idea and on the
Gibbs-Bogoliubov-Feynman variational principle, especially in the range of low
temperatures.Comment: 15 pages, 5 figures, one more section adde
Computational Study of the Structure and Thermodynamic Properties of Ammonium Chloride Clusters Using a Parallel J-Walking Approach
The thermodynamic and structural properties of (NHCl) clusters,
n=3-10 are studied. Using the method of simulated annealing, the geometries of
several isomers for each cluster size are examined. Jump-walking Monte Carlo
simulations are then used to compute the constant-volume heat capacity for each
cluster size over a wide temperature range. To carry out these simulations a
new parallel algorithm is developed using the Parallel Virtual Machine (PVM)
software package. Features of the cluster potential energy surfaces, such as
energy differences among isomers and rotational barriers of the ammonium ions,
are found to play important roles in determining the shape of the heat capacity
curves.Comment: Journal of Chemical Physics, accepted for publicatio
Ground-state properties of rutile: electron-correlation effects
Electron-correlation effects on cohesive energy, lattice constant and bulk
compressibility of rutile are calculated using an ab-initio scheme. A
competition between the two groups of partially covalent Ti-O bonds is the
reason that the correlation energy does not change linearly with deviations
from the equilibrium geometry, but is dominated by quadratic terms instead. As
a consequence, the Hartree-Fock lattice constants are close to the experimental
ones, while the compressibility is strongly renormalized by electronic
correlations.Comment: 1 figure to appear in Phys. Rev.
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