643 research outputs found
Рудная минерализация Агардагского ультрамафитового массива (Республика Тыва)
Исследованы особенности рудной минерализации Агардагского ультрамафитового массива, расположенного в юго-восточной части Республики Тыва. Показаны типоморфизм и химический состав минералов. Полученные данные позволили установить степень частичного плавления исходного субстрата и температуру его метаморфического преобразования при перемещении и консолидации в земной коре. Удалось проследить эволюционную направленность изменения химического состава хромшпинелидов и ассоциирующих с ними сульфидов, которая определяется условиями их деплетирования в верхней мантии и последующими метаморфогенными преобразованиями
KineticNet: Deep learning a transferable kinetic energy functional for orbital-free density functional theory
Orbital-free density functional theory (OF-DFT) holds the promise to compute
ground state molecular properties at minimal cost. However, it has been held
back by our inability to compute the kinetic energy as a functional of the
electron density only. We here set out to learn the kinetic energy functional
from ground truth provided by the more expensive Kohn-Sham density functional
theory. Such learning is confronted with two key challenges: Giving the model
sufficient expressivity and spatial context while limiting the memory footprint
to afford computations on a GPU; and creating a sufficiently broad distribution
of training data to enable iterative density optimization even when starting
from a poor initial guess. In response, we introduce KineticNet, an equivariant
deep neural network architecture based on point convolutions adapted to the
prediction of quantities on molecular quadrature grids. Important contributions
include convolution filters with sufficient spatial resolution in the vicinity
of the nuclear cusp, an atom-centric sparse but expressive architecture that
relays information across multiple bond lengths; and a new strategy to generate
varied training data by finding ground state densities in the face of
perturbations by a random external potential. KineticNet achieves, for the
first time, chemical accuracy of the learned functionals across input densities
and geometries of tiny molecules. For two electron systems, we additionally
demonstrate OF-DFT density optimization with chemical accuracy.Comment: 10 pages, 8 figure
Importance of electronic self-consistency in the TDDFT based treatment of nonadiabatic molecular dynamics
A mixed quantum-classical approach to simulate the coupled dynamics of
electrons and nuclei in nanoscale molecular systems is presented. The method
relies on a second order expansion of the Lagrangian in time-dependent density
functional theory (TDDFT) around a suitable reference density. We show that the
inclusion of the second order term renders the method a self-consistent scheme
and improves the calculated optical spectra of molecules by a proper treatment
of the coupled response. In the application to ion-fullerene collisions, the
inclusion of self-consistency is found to be crucial for a correct description
of the charge transfer between projectile and target. For a model of the
photoreceptor in retinal proteins, nonadiabatic molecular dynamics simulations
are performed and reveal problems of TDDFT in the prediction of intra-molecular
charge transfer excitations.Comment: 9 pages, 8 figures. Minor changes in content wrt older versio
Time-dependent density-functional and reduced density-matrix methods for few electrons: Exact versus adiabatic approximations
To address the impact of electron correlations in the linear and non-linear
response regimes of interacting many-electron systems exposed to time-dependent
external fields, we study one-dimensional (1D) systems where the interacting
problem is solved exactly by exploiting the mapping of the 1D -electron
problem onto an -dimensional single electron problem. We analyze the
performance of the recently derived 1D local density approximation as well as
the exact-exchange orbital functional for those systems. We show that the
interaction with an external resonant laser field shows Rabi oscillations which
are detuned due to the lack of memory in adiabatic approximations. To
investigate situations where static correlations play a role, we consider the
time-evolution of the natural occupation numbers associated to the reduced
one-body density matrix. Those studies shed light on the non-locality and
time-dependence of the exchange and correlation functionals in time-dependent
density and density-matrix functional theories.Comment: 19 pages, 13 figures, version as published apart from layou
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Quinoidal Azaacenes: 99 % Diradical Character
Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low-energy gap ΔEST′ of 0.58 to 1.0 kcal mol−1. The species are persistent in solution (half-life≈14–21 h) and in the solid state they are stable for weeks
Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study
Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their
crystalline state are among the most promising materials for organic
opto-electronics. Following previous works on oligoacenes we present a
systematic comparative study of the electronic, optical, and transport
properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using
density functional theory (DFT) and time-dependent DFT we computed: (i)
electron affinities and first ionization energies; (ii) quasiparticle
correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied
molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv)
electronic absorption spectra of neutral and charged systems. The
excitonic effects are estimated by comparing the optical gap and the
quasiparticle corrected HOMO-LUMO energy gap. For each molecular property
computed, general trends as a function of molecular size and charge state are
discussed. Overall, we find that circumacenes have the best transport
properties, displaying a steeper decrease of the molecular reorganization
energy at increasing sizes, while oligorylenes are much more efficient in
absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical
Physics (14/04/2011
Electron attachment to valence-excited CO
The possibility of electron attachment to the valence state of CO
is examined using an {\it ab initio} bound-state multireference configuration
interaction approach. The resulting resonance has symmetry;
the higher vibrational levels of this resonance state coincide with, or are
nearly coincident with, levels of the parent state. Collisional
relaxation to the lowest vibrational levels in hot plasma situations might
yield the possibility of a long-lived CO state.Comment: Revtex file + postscript file for one figur
Optical absorption in boron clusters B and B : A first principles configuration interaction approach
The linear optical absorption spectra in neutral boron cluster B and
cationic B are calculated using a first principles correlated
electron approach. The geometries of several low-lying isomers of these
clusters were optimized at the coupled-cluster singles doubles (CCSD) level of
theory. With these optimized ground-state geometries, excited states of
different isomers were computed using the singles configuration-interaction
(SCI) approach. The many body wavefunctions of various excited states have been
analysed and the nature of optical excitation involved are found to be of
collective, plasmonic type.Comment: 22 pages, 38 figures. An invited article submitted to European
Physical Journal D. This work was presented in the International Symposium on
Small Particles and Inorganic Clusters - XVI, held in Leuven, Belgiu
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