1,730 research outputs found
Chemical application of diffusion quantum Monte Carlo
The diffusion quantum Monte Carlo (QMC) method gives a stochastic solution to the Schroedinger equation. This approach is receiving increasing attention in chemical applications as a result of its high accuracy. However, reducing statistical uncertainty remains a priority because chemical effects are often obtained as small differences of large numbers. As an example, the single-triplet splitting of the energy of the methylene molecule CH sub 2 is given. The QMC algorithm was implemented on the CYBER 205, first as a direct transcription of the algorithm running on the VAX 11/780, and second by explicitly writing vector code for all loops longer than a crossover length C. The speed of the codes relative to one another as a function of C, and relative to the VAX, are discussed. The computational time dependence obtained versus the number of basis functions is discussed and this is compared with that obtained from traditional quantum chemistry codes and that obtained from traditional computer architectures
Inelastic Collisions in an Ultracold quasi-2D Gas
We present a formalism for rigorous calculations of cross sections for
inelastic and reactive collisions of ultracold atoms and molecules confined by
laser fields in quasi-2D geometry. Our results show that the
elastic-to-inelastic ratios of collision cross sections are enhanced in the
presence of a laser confinement and that the threshold energy dependence of the
collision cross sections can be tuned by varying the confinement strength and
external magnetic fields. The enhancement of the elastic-to-inelastic ratios is
inversely proportional to , where is
the kinetic energy and is the oscillation frequency of the trapped
particles in the confinement potential.Comment: 4 pages, 4 figure
Total angular momentum representation for atom-molecule collisions in electric fields
It is shown that the atom-molecule collision problem in the presence of an
external electric field can be solved using the total angular momentum
representation in the body-fixed coordinated frame, leading to a
computationally efficient method for ab initio modeling of low-temperature
scattering phenomena. Our calculations demonstrate rapid convergence of the
cross sections for vibrational and Stark relaxation in He-CaD collisions with
the number of total angular momentum states in the basis set, leading to a
5-100 fold increase in computational efficiency over the previously used
methods based on the fully uncoupled space-fixed representation. These results
open up the possibility of carrying out numerically converged quantum
scattering calculations on a wide array of atom-molecule collisions and
chemical reactions in the presence of electric fields.Comment: 19 pages, 3 figures, 1 tabl
All-electron quantum Monte Carlo calculations for the noble gas atoms He to Xe
We report all-electron variational and diffusion quantum Monte Carlo (VMC and
DMC) calculations for the noble gas atoms He, Ne, Ar, Kr, and Xe. The
calculations were performed using Slater-Jastrow wave functions with
Hartree-Fock single-particle orbitals. The quality of both the optimized
Jastrow factors and the nodal surfaces of the wave functions declines with
increasing atomic number Z, but the DMC calculations are tractable and well
behaved in all cases. We discuss the scaling of the computational cost of DMC
calculations with Z
Explicitly correlated trial wave functions in Quantum Monte Carlo calculations of excited states of Be and Be-
We present a new form of explicitly correlated wave function whose parameters
are mainly linear, to circumvent the problem of the optimization of a large
number of non-linear parameters usually encountered with basis sets of
explicitly correlated wave functions. With this trial wave function we
succeeded in minimizing the energy instead of the variance of the local energy,
as is more common in quantum Monte Carlo methods. We applied this wave function
to the calculation of the energies of Be 3P (1s22p2) and Be- 4So (1s22p3) by
variational and diffusion Monte Carlo methods. The results compare favorably
with those obtained by different types of explicitly correlated trial wave
functions already described in the literature. The energies obtained are
improved with respect to the best variational ones found in literature, and
within one standard deviation from the estimated non-relativistic limitsComment: 19 pages, no figures, submitted to J. Phys.
AB INITIO STUDY OF THE VIBRATIONAL SPECTRA OF NO3
The vibrational spectra and geometry of the NO, molecule is studied using ab initio SCF and CASSCF methods. For all levels of theory and basis set the highest symmetry found is Czv, Vibrational levels agree well with recent experimental results
SUSY parameter determination at the LHC using cross sections and kinematic edges
We study the determination of supersymmetric parameters at the LHC from a
global fit including cross sections and edges of kinematic distributions. For
illustration, we focus on a minimal supergravity scenario and discuss how well
it can be constrained at the LHC operating at 7 and 14 TeV collision energy,
respectively. We find that the inclusion of cross sections greatly improves the
accuracy of the SUSY parameter determination, and allows to reliably extract
model parameters even in the initial phase of LHC data taking with 7 TeV
collision energy and 1/fb integrated luminosity. Moreover, cross section
information may be essential to study more general scenarios, such as those
with non-universal gaugino masses, and distinguish them from minimal,
universal, models.Comment: 22 pages, 8 figure
Changes in Scottish suicide rates during the Second World War
BACKGROUND: It is believed that total reported suicide rates tend to decrease during wartime. However, analysis of suicide rates during recent conflicts suggests a more complex picture, with increases in some age groups and changes in method choice. As few age and gender specific analyses of more distant conflicts have been conducted, it is not clear if these findings reflect a change in the epidemiology of suicide in wartime. Therefore, we examined suicide rates in Scotland before, during and after the Second World War to see if similar features were present. METHODS: Data on deaths in Scotland recorded as suicide during the period 1931 – 1952, and population estimates for each of these years, were obtained from the General Register Office for Scotland. Using computer spreadsheets, suicide rates by gender, age and method were calculated. Forward stepwise logistic regression was used to assess the effect of gender, war and year on suicide rates using SAS V8.2. RESULTS: The all-age suicide rate among both men and women declined during the period studied. However, when this long-term decline is taken into account, the likelihood of suicide during the Second World War was higher than during both the pre-War and post-War periods. Suicide rates among men aged 15–24 years rose during the Second World War, peaking at 148 per million (41 deaths) during 1942 before declining to 39 per million (10 deaths) by 1945, while the rate among men aged 25–34 years reached 199 per million (43 deaths) during 1943 before falling to 66 per million (23 deaths) by 1946. This was accompanied by an increase in male suicides attributable to firearms and explosives during the War years which decreased following its conclusion. CONCLUSION: All age male and female suicide rates decreased in Scotland during World War II. However, once the general background decrease in suicide rates over the whole period is accounted for, the likelihood of suicide among the entire Scottish population during the Second World War was elevated. The overall decrease in suicide rates concealed large increases in younger male age groups during the War years, and an increase in male suicides recorded as due to the use of firearms. We conclude that the effects of war on younger people, reported in recent conflicts in Central Europe, were also seen in Scotland during the Second World War. The results support the findings of studies of recent conflicts which have found a heterogeneous picture with respect to age specific suicide rates during wartime
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