382 research outputs found
Brief review related to the foundations of time-dependent density functional theory
The electron density n(\rb,t), which is the central tool of time-dependent
density functional theory, is presently considered to be derivable from a
one-body time-dependent potential V(\rb,t), via one-electron wave functions
satisfying a time- dependent Schr\"{o}dinger equation. This is here related via
a generalized equation of motion to a Dirac density matrix now involving .
Linear response theory is then surveyed, with a special emphasis on the
question of causality with respect to the density dependence of the potential.
Extraction of V(\rb,t) for solvable models is also proposed
Higher harmonics and ac transport from time dependent density functional theory
We report on dynamical quantum transport simulations for realistic molecular
devices based on an approximate formulation of time-dependent Density
Functional Theory with open boundary conditions. The method allows for the
computation of various properties of junctions that are driven by alternating
bias voltages. Besides the ac conductance for hexene connected to gold leads
via thiol anchoring groups, we also investigate higher harmonics in the current
for a benzenedithiol device. Comparison to a classical quasi-static model
reveals that quantum effects may become important already for small ac bias and
that the full dynamical simulations exhibit a much lower number of higher
harmonics. Current rectification is also briefly discussed.Comment: submitted to J. Comp. Elec. (special issue
Implementation and benchmark of a long-range corrected functional in the density functional based tight-binding method
Bridging the gap between first principles methods and empirical schemes, the
density functional based tight-binding method (DFTB) has become a versatile
tool in predictive atomistic simulations over the past years. One of the major
restrictions of this method is the limitation to local or gradient corrected
exchange-correlation functionals. This excludes the important class of hybrid
or long-range corrected functionals, which are advantageous in thermochemistry,
as well as in the computation of vibrational, photoelectron and optical
spectra. The present work provides a detailed account of the implementation of
DFTB for a long-range corrected functional in generalized Kohn-Sham theory. We
apply the method to a set of organic molecules and compare ionization
potentials and electron affinities with the original DFTB method and higher
level theory. The new scheme cures the significant overpolarization in electric
fields found for local DFTB, which parallels the functional dependence in first
principles density functional theory (DFT). At the same time the computational
savings with respect to full DFT calculations are not compromised as evidenced
by numerical benchmark data
ELUCIDATING THE BIOCHEMICAL WIZARDRY OF TRITERPENE METABOLISM IN \u3ci\u3eBOTROYCOCCUS BRAUNII\u3c/i\u3e
B. braunii is a green alga that has attracted attention as a potential renewable fuel source due to its high oil content and the archeological record of its unique contribution to oil and coal shales. Three extant chemotypes of B. braunii have been described, namely race A, race B, and race L, which accumulate alkadienes and alkatrienes, botryococcene and squalene and their methylated derivatives, and lycopadiene, respectively. The methylated triterpenes, particularly botryococcenes, produced by race B can be efficiently converted to high quality combustible fuels and other petrochemicals; however, botryococcene biosynthesis has remained enigmatic.
It has been suggested that botryococcene biosynthesis could resemble that of squalene, arising from an initial condensation of two molecules of farnesyl diphosphate (FPP) to form pre-squalene diphosphate (PSPP), which then undergoes a reductive rearrangement to form squalene, or in an alternative reductive rearrangement, botryococcene. Based on the proposed similarities, we predicted that a botryococcene synthase would resemble squalene synthase and hence, isolated squalene synthase-like genes from B. braunii race B. While B. braunii does harbor at least one typical squalene synthase, none of the other three squalene synthase-like (SSL) genes encode for botryococcene biosynthesis directly. SSL-1 catalyzes the biosynthesis of PSPP and SSL-2 the biosynthesis of bisfarnesyl ether and to a lesser extent squalene, while SSL-3 does not appear able to directly utilize FPP as a substrate. However, when SSL-1 is combined with either SSL-2 or SSL-3, in vivo and in vitro, robust squalene or botryococcene biosynthesis was observed, respectively. These findings were unexpected because squalene synthase, an ancient and likely progenitor to the other Botryococcus triterpene synthases, catalyzes a two-step reaction within a single enzyme unit without intermediate release, yet in B. braunii, these activities appear to have separated and evolved inter-dependently for specialized triterpene production. Expression of various configurations of the SSL genes in TN-7 yeast demonstrates that botryococcene can be efficiently produced in a heterologous host.
Additionally, three triterpene methyltransferase (TMTs) were isolated which efficiently catalyze the transfer of a methyl group from S-adenosyl methionine (SAM) to either squalene (TMT-1 and TMT-2) or botryococcene (TMT-3) in vivo and in vitro. Co-expression of the various TMT genes with either squalene synthase or botryococcene synthase in TN-7 yeast resulted in the accumulation of C31 and C32 methyl derivatives of squalene or botryococcene, demonstrating their potential for heterologous production. The methylation sites were determined by NMR spectroscopy to be identical to C31 and C32 methyl-derivatives of squalene or botryococcene observed in B. braunii race B.
Expression studies of various heterologous squalene synthase genes in S. cerevisiae corroborated an earlier but surprising observation reported in the literature. While the squalene synthase gene of S. cerevisiae was able to complement an erg9 (squalene synthase) knockout in yeast, squalene synthase genes from plants and animals were not. Chemical profiles revealed that squalene accumulated to significant levels in yeast expressing the squalene synthase of plant, animal, or S. cerevisiae. This suggested that it was not the ability of these heterologous synthase enzymes to produce squalene, but their inability to feed squalene into the native sterol biosynthetic pathway that prevented them from restoring normal ergosterol biosynthesis in S. cerevisiae. By examining the ability of chimera squalene synthase enzymes to complement the erg9 mutation, a discrete sequence of amino acids near the C-terminus of the enzyme was identified which is necessary and sufficient for allowing any squalene synthase to restore normal sterol metabolism
Effect of line defects on the electrical transport properties of monolayer MoS sheet
We present a computational study on the impact of line defects on the
electronic properties of monolayer MoS2. Four different kinds of line defects
with Mo and S as the bridging atoms, consistent with recent theoretical and
experimental observations are considered herein. We employ the density
functional tight-binding (DFTB) method with a Slater-Koster type DFTB-CP2K
basis set for evaluating the material properties of perfect and the various
defective MoS2 sheets. The transmission spectra is computed with a
DFTB-Non-Equilibrium Greens Function (NEGF) formalism. We also perform a
detailed analysis of the carrier transmission pathways under a small bias and
investigate the phase shifts in the transmission eigenstates of the defective
MoS2 sheets. Our simulations show a 2-4 folds decrease in carrier conductance
of MoS2 sheets in the presence of line defects as compared to that for the
perfect sheet
Time-dependent versus static quantum transport simulations beyond linear response
To explore whether the density-functional theory non-equilibrium Green's
function formalism (DFT-NEGF) provides a rigorous framework for quantum
transport, we carried out time-dependent density functional theory (TDDFT)
calculations of the transient current through two realistic molecular devices,
a carbon chain and a benzenediol molecule inbetween two aluminum electrodes.
The TDDFT simulations for the steady state current exactly reproduce the
results of fully self-consistent DFT-NEGF calculations even beyond linear
response. In contrast, sizable differences are found with respect to an
equilibrium, non-self-consistent treatment which are related here to
differences in the Kohn-Sham and fully interacting susceptibility of the device
region. Moreover, earlier analytical conjectures on the equivalence of static
and time-dependent approaches in the low bias regime are confirmed with high
numerical precision.Comment: 4 pages, 4 figure
Towards a simplified description of thermoelectric materials: Accuracy of approximate density functional theory for phonon dispersions
We calculate the phonon-dispersion relations of several two-dimensional
materials and diamond using the density-functional based tight-binding approach
(DFTB). Our goal is to verify if this numerically efficient method provides
sufficiently accurate phonon frequencies and group velocities to compute
reliable thermoelectric properties. To this end, the results are compared to
available DFT results and experimental data. To quantify the accuracy for a
given band, a descriptor is introduced that summarizes contributions to the
lattice conductivity that are available already in the harmonic approximation.
We find that the DFTB predictions depend strongly on the employed repulsive
pair-potentials, which are an important prerequisite of this method. For
carbon-based materials, accurate pair-potentials are identified and lead to
errors of the descriptor that are of the same order as differences between
different local and semi-local DFT approaches
Optimal Detection for Diffusion-Based Molecular Timing Channels
This work studies optimal detection for communication over diffusion-based
molecular timing (DBMT) channels. The transmitter simultaneously releases
multiple information particles, where the information is encoded in the time of
release. The receiver decodes the transmitted information based on the random
time of arrival of the information particles, which is modeled as an additive
noise channel. For a DBMT channel without flow, this noise follows the L\'evy
distribution. Under this channel model, the maximum-likelihood (ML) detector is
derived and shown to have high computational complexity. It is also shown that
under ML detection, releasing multiple particles improves performance, while
for any additive channel with -stable noise where (such as
the DBMT channel), under linear processing at the receiver, releasing multiple
particles degrades performance relative to releasing a single particle. Hence,
a new low-complexity detector, which is based on the first arrival (FA) among
all the transmitted particles, is proposed. It is shown that for a small number
of released particles, the performance of the FA detector is very close to that
of the ML detector. On the other hand, error exponent analysis shows that the
performance of the two detectors differ when the number of released particles
is large.Comment: 16 pages, 9 figures. Submitted for publicatio
Liquid-solid slip on charged walls: dramatic impact of charge distribution
Nanofluidic systems show great promises for applications in energy
conversion, where their performance can be enhanced by nanoscale liquid-solid
slip. However, efficiency is also controlled by surface charge, which is known
to reduce slip. Combining molecular dynamics simulations and analytical
developments, we show the dramatic impact of surface charge distribution on the
slip-charge coupling. Homogeneously charged graphene exhibits a very favorable
slip-charge relation (rationalized with a new theoretical model correcting some
weaknesses of the existing ones), leading to giant electrokinetic energy
conversion. In contrast, slip is strongly affected on heterogeneously charged
surfaces, due to the viscous drag induced by counter-ions trapped on the
surface. In that case slip should depend on the detailed physical chemistry of
the interface controlling the fraction of bound ions. Our numerical results and
theoretical models provide new fundamental insight on the molecular mechanisms
of liquid-solid slip, and practical guidelines for searching new functional
interfaces with optimal energy conversion properties, e.g. for blue energy or
waste heat harvesting.Comment: Main text: 7 pages, 3 figures; supplemental material: 22 pages, 5
figures; to be published in Physical Review Letter
- …