336 research outputs found
Density Functional Theory Calculation
Density functional theory (DFT) is a computational quantum mechanical modeling used in physics, chemistry, and materials science to evaluate the electronic structure (or nuclear structure, principally the ground state) of many-body systems, in particular atoms, molecules, and the condensed phases. A many-electron system's properties can be determined using functionals, i.e., functions of another function. In the case of DFT, these are functionals of the spatially dependent electron density. DFT is among the most popular and versatile methods available in condensed-matter physics, computational physics, and computational chemistry
Computing the Primordial Power Spectra Directly
The tree order power spectra of primordial inflation depend upon the
norm-squared of mode functions which oscillate for early times and then freeze
in to constant values. We derive simple differential equations for the power
spectra, that avoid the need to numerically simulate the physically irrelevant
phases of the mode functions. We also derive asymptotic expansions which should
be valid until a few e-foldings before first horizon crossing, thereby avoiding
the need to evolve mode functions from the ultraviolet over long periods of
inflation.Comment: 11 pages, uses LaTex2
Parity Effects in Spin Decoherence
We demonstrate that decoherence of many-spin systems can drastically differ
from decoherence of single-spin systems. The difference originates at the most
basic level, being determined by parity of the central system, i.e. by whether
the system comprises even or odd number of spin-1/2 entities. Therefore, it is
very likely that similar distinction between the central spin systems of even
and odd parity is important in many other situations. Our consideration
clarifies the physical origin of the unusual two-step decoherence found
previously in the two-spin systems.Comment: RevTeX 4, 5 pages, 2 figures; acknowledgments added; replaced with
the published version; journal reference adde
Improving the Single Scalar Consistency Relation
We propose a test of single-scalar inflation based on using the well-measured
scalar power spectrum to reconstruct the tensor power spectrum, up to a single
integration constant. Our test is a sort of integrated version of the
single-scalar consistency relation. This sort of test can be used effectively,
even when the tensor power spectrum is measured too poorly to resolve the
tensor spectral index. We give an example using simulated data based on a
hypothetical detection with tensor-to-scalar ratio . Our test can
also be employed for correlating scalar and tensor features in the far future
when the data is good.Comment: 16 pages, 1 figure, uses LaTeX2e version 2 extensively revised for
publicatio
Lattice Boltzmann modeling of water-like fluids
We review recent advances on the mesoscopic modeling of water-like fluids,based on the lattice Boltzmann (LB) methodology.The main idea is to enrich the basic LB (hydro)-dynamics with angular degrees of freedom responding to suitable directional potentials between water-like molecules.The model is shown to reproduce some microscopic features of liquid water, such as an average number of hydrogen bonds per molecules (HBs) between and , as well as a qualitatively correctstatistics of the hydrogen bond angle as a function of the temperature.Future developments, based on the coupling the present water-like LB model with the dynamics of suspended bodies,such as biopolymers, may open new angles of attack to the simulation of complex biofluidic problems, such as protein folding and aggregation, and the motion of large biomolecules in complex cellular environments
The role of a form of vector potential - normalization of the antisymmetric gauge
Results obtained for the antisymmetric gauge A=[Hy,-Hx]/2 by Brown and Zak
are compared with those based on pure group-theoretical considerations and
corresponding to the Landau gauge A=[0,Hx]. Imposing the periodic boundary
conditions one has to be very careful since the first gauge leads to a factor
system which is not normalized. A period N introduced in Brown's and Zak's
papers should be considered as a magnetic one, whereas the crystal period is in
fact 2N. The `normalization' procedure proposed here shows the equivalence of
Brown's, Zak's, and other approaches. It also indicates the importance of the
concept of magnetic cells. Moreover, it is shown that factor systems (of
projective representations and central extensions) are gauge-dependent, whereas
a commutator of two magnetic translations is gauge-independent. This result
indicates that a form of the vector potential (a gauge) is also important in
physical investigations.Comment: RevTEX, 9 pages, to be published in J. Math. Phy
A numerical projection technique for large-scale eigenvalue problems
We present a new numerical technique to solve large-scale eigenvalue
problems. It is based on the projection technique, used in strongly correlated
quantum many-body systems, where first an effective approximate model of
smaller complexity is constructed by projecting out high energy degrees of
freedom and in turn solving the resulting model by some standard eigenvalue
solver.
Here we introduce a generalization of this idea, where both steps are
performed numerically and which in contrast to the standard projection
technique converges in principle to the exact eigenvalues. This approach is not
just applicable to eigenvalue problems encountered in many-body systems but
also in other areas of research that result in large scale eigenvalue problems
for matrices which have, roughly speaking, mostly a pronounced dominant
diagonal part. We will present detailed studies of the approach guided by two
many-body models.Comment: 7 pages, 4 figure
An efficient scheme for numerical simulations of the spin-bath decoherence
We demonstrate that the Chebyshev expansion method is a very efficient
numerical tool for studying spin-bath decoherence of quantum systems. We
consider two typical problems arising in studying decoherence of quantum
systems consisting of few coupled spins: (i) determining the pointer states of
the system, and (ii) determining the temporal decay of quantum oscillations. As
our results demonstrate, for determining the pointer states, the
Chebyshev-based scheme is at least a factor of 8 faster than existing
algorithms based on the Suzuki-Trotter decomposition. For the problems of
second type, the Chebyshev-based approach has been 3--4 times faster than the
Suzuki-Trotter-based schemes. This conclusion holds qualitatively for a wide
spectrum of systems, with different spin baths and different Hamiltonians.Comment: 8 pages (RevTeX), 3 EPS figure
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