107 research outputs found
N-representability of two-electron densities and density matrices and the application to the few-body problem
We have found a (dense) basis for the N-representable, two-electron
densities, in which all N-representable two-electron densities can be expanded,
using positive coefficients. The inverse problem of finding a representative
wavefunction, giving a prescribed two-electron density, has also been solved.
The two-electron densities are found to lie in a convex set in a vector space.
We show that density matrices are more complicated objects than densities, and
density matrices do not seem to lie in a convex set. An algorithm to compute
the ground-state energy of a few-particle system is proposed, based on the
obtained results, where the correlation is treated exactly
The elastic constants of MgSiO3 perovskite at pressures and temperatures of the Earth's mantle
The temperature anomalies in the Earth's mantle associated with thermal
convection1 can be inferred from seismic tomography, provided that the elastic
properties of mantle minerals are known as a function of temperature at mantle
pressures. At present, however, such information is difficult to obtain
directly through laboratory experiments. We have therefore taken advantage of
recent advances in computer technology, and have performed finite-temperature
ab initio molecular dynamics simulations of the elastic properties of MgSiO3
perovskite, the major mineral of the lower mantle, at relevant thermodynamic
conditions. When combined with the results from tomographic images of the
mantle, our results indicate that the lower mantle is either significantly
anelastic or compositionally heterogeneous on large scales. We found the
temperature contrast between the coldest and hottest regions of the mantle, at
a given depth, to be about 800K at 1000 km, 1500K at 2000 km, and possibly over
2000K at the core-mantle boundary.Comment: Published in: Nature 411, 934-937 (2001
Computational Complexity in Electronic Structure
In quantum chemistry, the price paid by all known efficient model chemistries
is either the truncation of the Hilbert space or uncontrolled approximations.
Theoretical computer science suggests that these restrictions are not mere
shortcomings of the algorithm designers and programmers but could stem from the
inherent difficulty of simulating quantum systems. Extensions of computer
science and information processing exploiting quantum mechanics has led to new
ways of understanding the ultimate limitations of computational power.
Interestingly, this perspective helps us understand widely used model
chemistries in a new light. In this article, the fundamentals of computational
complexity will be reviewed and motivated from the vantage point of chemistry.
Then recent results from the computational complexity literature regarding
common model chemistries including Hartree-Fock and density functional theory
are discussed.Comment: 14 pages, 2 figures, 1 table. Comments welcom
Transition-metal interactions in aluminum-rich intermetallics
The extension of the first-principles generalized pseudopotential theory
(GPT) to transition-metal (TM) aluminides produces pair and many-body
interactions that allow efficient calculations of total energies. In
aluminum-rich systems treated at the pair-potential level, one practical
limitation is a transition-metal over-binding that creates an unrealistic TM-TM
attraction at short separations in the absence of balancing many-body
contributions. Even with this limitation, the GPT pair potentials have been
used effectively in total-energy calculations for Al-TM systems with TM atoms
at separations greater than 4 AA. An additional potential term may be added for
systems with shorter TM atom separations, formally folding repulsive
contributions of the three- and higher-body interactions into the pair
potentials, resulting in structure-dependent TM-TM potentials. Towards this
end, we have performed numerical ab-initio total-energy calculations using VASP
(Vienna Ab Initio Simulation Package) for an Al-Co-Ni compound in a particular
quasicrystalline approximant structure. The results allow us to fit a
short-ranged, many-body correction of the form a(r_0/r)^{b} to the GPT pair
potentials for Co-Co, Co-Ni, and Ni-Ni interactions.Comment: 18 pages, 5 figures, submitted to PR
Modeling the actinides with disordered local moments
A first-principles disordered local moment (DLM) picture within the
local-spin-density and coherent potential approximations (LSDA+CPA) of the
actinides is presented. The parameter free theory gives an accurate description
of bond lengths and bulk modulus. The case of -Pu is studied in
particular and the calculated density of states is compared to data from
photo-electron spectroscopy. The relation between the DLM description, the
dynamical mean field approach and spin-polarized magnetically ordered modeling
is discussed.Comment: 6 pages, 4 figure
Brownian motion: a paradigm of soft matter and biological physics
This is a pedagogical introduction to Brownian motion on the occasion of the
100th anniversary of Einstein's 1905 paper on the subject. After briefly
reviewing Einstein's work in its contemporary context, we pursue some lines of
further developments and applications in soft condensed matter and biology.
Over the last century Brownian motion became promoted from an odd curiosity of
marginal scientific interest to a guiding theme pervading all of the modern
(live) sciences.Comment: 30 pages, revie
The effect of silicon impurities on the phase diagram of iron and possible implications for the Earth's core structure
AIDS-related mycoses: the way forward.
The contribution of fungal infections to the morbidity and mortality of HIV-infected individuals is largely unrecognized. A recent meeting highlighted several priorities that need to be urgently addressed, including improved epidemiological surveillance, increased availability of existing diagnostics and drugs, more training in the field of medical mycology, and better funding for research and provision of treatment, particularly in developing countries
The interplay between employee and firm customer orientation: substitution effect and the contingency role of performance-related rewards
This paper identifies and explains a potential tension between a firm's emphasis on customer orientation (CO) and the extent to which employees value CO as a success factor for individual performance. Based on selfâdetermination theory and CO implementation research, the authors propose that firm CO may represent both autonomous and controlled motivations for CO, but that employeesâ CO is more strongly linked to individual performance when employees experience solely autonomous motivation. Hence, the authors expect a substitution effect whereby the link between employeesâ CO and their performance is weaker when firm CO is high. Furthermore, the authors examine a boundary condition for the previous hypothesis and propose that performanceâcontingent rewards have a positive effect on the internalization of the extrinsic motivation stemming from firm CO. Two multilevel studies with 979 employees and 201 top management team members from 132 firms support these hypotheses. Against previous research, these findings offer a new perspective on the effectiveness of CO initiatives, propose employeesâ motivational states as the theoretical explanation for the heterogeneity in the link between employee CO and performance, and reappraise the role of performanceâcontingent rewards in CO research. Managerial implications for the effective implementation of customerâoriented initiatives within firms are provided
The Interplay Between Employee and Firm Customer Orientation: Substitution Effect and the Contingency Role of Performance-Related Rewards
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