1,212 research outputs found
Embedded-Cluster Calculations in a Numeric Atomic Orbital Density-Functional Theory Framework
We integrate the all-electron electronic structure code FHI-aims into the
general ChemShell package for solid-state embedding (QM/MM) calculations. A
major undertaking in this integration is the implementation of pseudopotential
functionality into FHI-aims to describe cations at the QM/MM boundary through
effective core potentials and therewith prevent spurious overpolarization of
the electronic density. Based on numeric atomic orbital basis sets, FHI-aims
offers particularly efficient access to exact exchange and second order
perturbation theory, rendering the established QM/MM setup an ideal tool for
hybrid and double-hybrid level DFT calculations of solid systems. We illustrate
this capability by calculating the reduction potential of Fe in the
Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for
(photo-)catalytic water oxidation at TiO2(110).Comment: 12 pages, 4 figure
Search for long-lived states in antiprotonic lithium
The spectrum of the (L_i^3 + p-bar + 2e) four-body system was calculated in
an adiabatic approach. The two-electron energies were approximated by a sum of
two single-electron effective charge two-center energies as suggested in [6].
While the structure of the spectrum does not exclude the existence of
long-lived states, their experimental observability is still to be clarified
Melting and evaporation transitions in small Al clusters: canonical Monte-Carlo simulations
A dimer of bound atoms cannot melt, only dissociate. Bulk metals show a well
defined first order transition between their solid and liquid phases. The
appearance of the melting transition is explored for increasing clusters sizes
via the signatures in the specific heat and the root mean square of the bond
lengths (Berry parameter) by means of Monte-Carlo simulations
of Al clusters modelled by Gupta potentials. Clear signatures of a melting
transition appear for atoms. Closed-shell effects are shown for
clusters with up to 56 atoms. The melting transition is compared in detail with
the dissociation transition, which induces a second and possibly much larger
local maximum in the specific heat at higher temperatures. Larger clusters are
shown to fragment into dimers and trimers, which in turn dissociate at higher
temperatures.Comment: 6 pages, 7 figure
Local Simulation Algorithms for Coulomb Interaction
Long ranged electrostatic interactions are time consuming to calculate in
molecular dynamics and Monte-Carlo simulations. We introduce an algorithmic
framework for simulating charged particles which modifies the dynamics so as to
allow equilibration using a local Hamiltonian. The method introduces an
auxiliary field with constrained dynamics so that the equilibrium distribution
is determined by the Coulomb interaction. We demonstrate the efficiency of the
method by simulating a simple, charged lattice gas.Comment: Last figure changed to improve demonstration of numerical efficienc
How does the 'ancient' asexual Philodina roseola (Rotifera:Bdelloidea) handle potential UVB-induced mutations?
Like other obligate asexuals, bdelloid rotifers are expected to suffer from degradation of their genomes through processes including the accumulation of deleterious mutations. However, sequence-based analyses in this regard remain inconclusive. Instead of looking for historical footprints of mutations in these ancient asexuals, we directly examined the susceptibility and ability to repair point mutations by the bdelloid Philodina roseola by inducing cyclobutane-pyrimidine dimers (CPDs) via exposure to UVB radiation (280-320 nm). For comparison, we performed analogous experiments with the facultative asexual monogonont rotifer Brachionus rubens. Different strategies were found for the two species. Philodina roseola appeared to shield itself from CPD induction through uncharacterized UV-absorbing compounds and, except for the genome reconstruction that occurs after desiccation, was largely unable to repair UVB-induced damage. By contrast, B. rubens was more susceptible to UVB irradiation, but could repair all induced damage in similar to 2 h. In addition, whereas UV irradiation had a significant negative impact on the reproductive output of P. roseola, and especially so after desiccation, that of B. rubens was unaffected. Although the strategy of P. roseola might suffice under natural conditions where UVB irradiation is less intense, the lack of any immediate CPD repair mechanisms in this species remains perplexing. It remains to be investigated how typical these results are for bdelloids as a group and therefore how reliant these animals are on desiccation-dependent genome repair to correct potential DNA damage given their obligate asexual lifestyle.</p
Alkali and Alkaline Earth Metal Compounds: Core-Valence Basis Sets and Importance of Subvalence Correlation
Core-valence basis sets for the alkali and alkaline earth metals Li, Be, Na,
Mg, K, and Ca are proposed. The basis sets are validated by calculating
spectroscopic constants of a variety of diatomic molecules involving these
elements. Neglect of correlation in K and Ca compounds will lead to
erratic results at best, and chemically nonsensical ones if chalcogens or
halogens are present. The addition of low-exponent functions to the K and
Ca basis sets is essential for smooth convergence of molecular properties.
Inclusion of inner-shell correlation is important for accurate spectroscopic
constants and binding energies of all the compounds. In basis set
extrapolation/convergence calculations, the explicit inclusion of alkali and
alkaline earth metal subvalence correlation at all steps is essential for K and
Ca, strongly recommended for Na, and optional for Li and Mg, while in Be
compounds, an additive treatment in a separate `core correlation' step is
probably sufficient. Consideration of inner-shell correlation energy in
first-row elements requires inclusion of `deep core' correlation
energy in K and Ca for consistency. The latter requires special CCVZ `deep
core correlation' basis sets. For compounds involving Ca bound to
electronegative elements, additional functions in the basis set are
strongly recommended. For optimal basis set convergence in such cases, we
suggest the sequence CV(D+3d)Z, CV(T+2d)Z, CV(Q+)Z, and CV5Z on calcium.Comment: Molecular Physics, in press (W. G. Richards issue); supplementary
material (basis sets in G98 and MOLPRO formats) available at
http://theochem.weizmann.ac.il/web/papers/group12.htm
Electron correlations for ground state properties of group IV semiconductors
Valence energies for crystalline C, Si, Ge, and Sn with diamond structure
have been determined using an ab-initio approach based on information from
cluster calculations. Correlation contributions, in particular, have been
evaluated in the coupled electron pair approximation (CEPA), by means of
increments obtained for localized bond orbitals and for pairs and triples of
such bonds. Combining these results with corresponding Hartree-Fock (HF) data,
we recover about 95 % of the experimental cohesive energies. Lattice constants
are overestimated at the HF level by about 1.5 %; correlation effects reduce
these deviations to values which are within the error bounds of this method. A
similar behavior is found for the bulk modulus: the HF values which are
significantly too high are reduced by correlation effects to about 97 % of the
experimental values.Comment: 22 pages, latex, 2 figure
Algorithm for numerical integration of the rigid-body equations of motion
A new algorithm for numerical integration of the rigid-body equations of
motion is proposed. The algorithm uses the leapfrog scheme and the quantities
involved are angular velocities and orientational variables which can be
expressed in terms of either principal axes or quaternions. Due to specific
features of the algorithm, orthonormality and unit norms of the orientational
variables are integrals of motion, despite an approximate character of the
produced trajectories. It is shown that the method presented appears to be the
most efficient among all known algorithms of such a kind.Comment: 4 pages, 1 figur
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Slow and fast single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line
Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single-photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single In(Ga)As quantum dot with its excitonic transition precisely set relative to the Cesium D1 transition. The delay of spectral components of the single-photon wave packet with almost Fourier-limited width is investigated in detail with a 200 MHz narrow-band monolithic Fabry-Pérot resonator. Reflecting the excited state hyperfine structure of Cesium, “slow light” and “fast light” behavior is observed. As a step towards room-temperature alkali vapor memories, quantum dot photons are delayed for 5 ns by strong dispersion between the two 1.17 GHz hyperfine-split excited state transitions. Based on optical pumping on the hyperfine-split ground states, we propose a simple, all-optically controllable delay for synchronization of heralded narrow-band photons in a quantum network
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