206 research outputs found
Optimal control of strong-field ionization with time-dependent density-functional theory
We show that quantum optimal control theory (OCT) and time-dependent
density-functional theory (TDDFT) can be combined to provide realistic
femtosecond laser pulses for an enhanced ionization yield in many-electron
systems. Using the H-molecule as a test case, the optimized laser pulse
from the numerically exact scheme is compared to pulses obtained from OCT+TDDFT
within the TD exact-exchange (TDEXX) and the TD local-density approximation
(TDLDA). We find that the TDDFT-pulses produces an ionization yield of up to
50% when applied to the exact system. In comparison, pulses with a single
frequency but the same fluence typically reach to yields around 5-15%, unless
the frequency is carefully tuned into a Fano-type resonance that leads to yield. On the other hand, optimization within the exact system alone leads
to yields higher than 80%, demonstrating that correlation effects beyond the
TDEXX and TDLDA can give rise to even more efficient ionization mechanisms
Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation
Self-consistent correlation potentials for H and LiH for various
inter-atomic separations are obtained within the random phase approximation
(RPA) of density functional theory. The RPA correlation potential shows a peak
at the bond midpoint, which is an exact feature of the true correlation
potential, but lacks another exact feature: the step important to preserve
integer charge on the atomic fragments in the dissociation limit. An analysis
of the RPA energy functional in terms of fractional charge is given which
confirms these observations. We find that the RPA misses the derivative
discontinuity at odd integer particle numbers but explicitly eliminates the
fractional spin error in the exact-exchange functional. The latter finding
explains the accurate total energy in the dissociation limit.Comment: 9 pages, 10 figure
Local anaesthetic cream for the alleviation of pain during venepuncture in Tanzanian schoolchidren
The analgesic effect and the usefulness of EMLA cream 5% in connection with venous blood-sampling was investigated in 42 Tanzanian schoolchildren. Approximately 2.5 g EMLA was applied to the right cubital fossa for a minimum of 120 min. The analgesic effect was pronounced - 93% of the venepunctures were pain-free and no child experienced severe pain. No adverse reactions were observed and the children could continue normal school work during the application time
The correlation potential in density functional theory at the GW-level: spherical atoms
As part of a project to obtain better optical response functions for nano
materials and other systems with strong excitonic effects we here calculate the
exchange-correlation (XC) potential of density-functional theory (DFT) at a
level of approximation which corresponds to the dynamically- screened-exchange
or GW approximation. In this process we have designed a new numerical method
based on cubic splines which appears to be superior to other techniques
previously applied to the "inverse engineering problem" of DFT, i.e., the
problem of finding an XC potential from a known particle density. The
potentials we obtain do not suffer from unphysical ripple and have, to within a
reasonable accuracy, the correct asymptotic tails outside localized systems.
The XC potential is an important ingredient in finding the particle-conserving
excitation energies in atoms and molecules and our potentials perform better in
this regard as compared to the LDA potential, potentials from GGA:s, and a DFT
potential based on MP2 theory.Comment: 13 pages, 9 figure
Discontinuities of the exchange-correlation kernel and charge-transfer excitations in time-dependent density functional theory
We identify the key property that the exchange-correlation (XC) kernel of
time-dependent density functional theory must have in order to describe
long-range charge-transfer excitations. We show that the discontinuity of the
XC potential as a function of particle number induces a space -and
frequency-dependent discontinuity of the XC kernel which diverges as
. In a combined donor-acceptor system, the same discontinuity
compensates for the vanishing overlap between the acceptor and donor orbitals,
thereby yielding a finite correction to the Kohn-Sham eigenvalue differences.
This mechanism is illustrated to first order in the Coulomb interaction.Comment: 6 pages, 3 figures (expanded version, accepted in Phys. Rev. A
Circannual variation in blood parasitism in a sub-Saharan migrant passerine bird, the garden warbler
Knowing the natural dynamics of pathogens in migratory birds is important, for example, to understand the factors that influence the transport of pathogens to and their transmission in new geographical areas, whereas the transmission of other pathogens might be restricted to a specific area. We studied haemosporidian blood parasites of the genera Plasmodium, Haemoproteus and Leucocytozoon in a migratory bird, the garden warbler Sylvia borin. Birds were sampled in spring, summer and early autumn at breeding grounds in Sweden, on migration at Capri, Italy and on arrival and departure from wintering staging areas in West Africa: mapping recoveries of garden warblers ringed in Fennoscandia and Capri showed that these sites are most probably on the migratory flyway of garden warblers breeding at Kvismaren. Overall, haemosporidian prevalence was 39%, involving 24 different parasite lineages. Prevalence varied significantly over the migratory cycle, with relatively high prevalence of blood parasites in the population on breeding grounds and at the onset of autumn migration, followed by marked declines in prevalence during migration both on spring and autumn passage. Importantly, we found that when examining circannual variation in the different lineages, significantly different prevalence profiles emerged both between and within genera. Our results suggest that differences in prevalence profiles are the result of either different parasite transmission strategies or coevolution between the host and the various parasite lineages. When separating parasites into common vs. rare lineages, we found that two peaks in the prevalence of rare parasites occur; on arrival at Swedish breeding grounds, and after the wintering period in Africa. Our results stress the importance of appropriate taxonomic resolution when examining host-parasite interactions, as variation in prevalence both between and within parasite genera can show markedly different patterns
Some open questions in TDDFT: Clues from Lattice Models and Kadanoff-Baym Dynamics
Two aspects of TDDFT, the linear response approach and the adiabatic local
density approximation, are examined from the perspective of lattice models. To
this end, we review the DFT formulations on the lattice and give a concise
presentation of the time-dependent Kadanoff-Baym equations, used to asses the
limitations of the adiabatic approximation in TDDFT. We present results for the
density response function of the 3D homogeneous Hubbard model, and point out a
drawback of the linear response scheme based on the linearized Sham-Schl\"uter
equation. We then suggest a prescription on how to amend it. Finally, we
analyze the time evolution of the density in a small cubic cluster, and compare
exact, adiabatic-TDDFT and Kadanoff-Baym-Equations densities. Our results show
that non-perturbative (in the interaction) adiabatic potentials can perform
quite well for slow perturbations but that, for faster external fields, memory
effects, as already present in simple many-body approximations, are clearly
required.Comment: 15 pages, submitted to Chemical Physic
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