348 research outputs found
Real-Time Propagation TDDFT and Density Analysis for Exciton Couplings Calculations in Large Systems
Photo-active systems are characterized by their capacity of absorbing light
energy and transforming it. Usually, more than one chromophore is involved in
the light absorption and excitation transport processes in complex systems.
Linear-Response Time-Dependent Density Functional (LR-TDDFT) is commonly used
to identify excitation energies and transition properties by solving well-known
Casida's equation for single molecules. However, this methodology is not useful
in practice when dealing with multichromophore systems. In this work, we extend
our local density decomposition method that enables to disentangle individual
contributions into the absorption spectrum to computation of exciton dynamic
properties, such as exciton coupling parameters. We derive an analytical
expression for the transition density from Real-Time Propagation TDDFT
(P-TDDFT) based on Linear Response theorems. We demonstrate the validity of our
method to determine transition dipole moments, transition densities and exciton
coupling for systems of increasing complexity. We start from the isolated
benzaldehyde molecule, perform a distance analysis for -stacked dimers and
finally map the exciton coupling for a 14 benzaldehyde cluster.Comment: 32 pages, 8 figures; added references in introductions, typos fixe
Nanotube-based scanning rotational microscope
A scheme of the scanning rotational microscope is designed. This scheme is
based on using carbon nanotubes simultaneously as a probe tip and as a bolt/nut
pair which converts translational displacements of two piezo actuators into
pure rotation of the probe tip. First-principles calculations of the
interaction energy between movable and rotational parts of the microscope
confirms the capability for its operation. The scanning rotational microscope
with a chemically functionalized nanotube-based tip can be used to study how
the interaction between individual molecules or a molecule and a surface
depends on their relative orientation.Comment: 4 pages, 3 figure
Dislocations in stacking and commensurate-incommensurate phase transition in bilayer graphene and hexagonal boron nitride
Dislocations corresponding to a change of stacking in two-dimensional
hexagonal bilayers, graphene and boron nitride, and associated with boundaries
between commensurate domains are investigated using the two-chain
Frenkel-Kontorova model on top of ab initio calculations. Structural
transformations of bilayers in which the bottom layer is stretched and the
upper one is left to relax freely are considered for gradually increased
elongation of the bottom layer. Formation energies of dislocations, dislocation
width and orientation of the boundary between commensurate domains are analyzed
depending on the magnitude and direction of elongation. The second-order phase
transition from the commensurate phase to the incommensurate one with multiple
dislocations is predicted to take place at some critical elongation. The order
parameter for this transition corresponds to the density of dislocations, which
grows continuously upon increasing the elongation of the bottom layer above the
critical value. In graphene and metastable boron nitride with the layers
aligned in the same direction, where elementary dislocations are partial, this
transition, however, is preceded by formation of the first dislocation at the
elongation smaller than the critical one. The phase diagrams including this
intermediate state are plotted in coordinates of the magnitude and direction of
elongation of the bottom layer.Comment: 15 pages, 9 figure
Transformation of amorphous carbon clusters to fullerenes
Transformation of amorphous carbon clusters into fullerenes under high
temperature is studied using molecular dynamics simulations at microsecond
times. Based on the analysis of both structure and energy of the system, it is
found that fullerene formation occurs in two stages. Firstly, fast
transformation of the initial amorphous structure into a hollow sp shell
with a few chains attached occurs with a considerable decrease of the potential
energy and the number of atoms belonging to chains and to the amorphous domain.
Then, insertion of remaining carbon chains into the sp network takes place
at the same time with the fullerene shell formation. Two types of defects
remaining after the formation of the fullerene shell are revealed: 7-membered
rings and single one-coordinated atoms. One of the fullerene structures
obtained contains no defects at all, which demonstrates that defect-free carbon
cages can be occasionally formed from amorphous precursors directly without
defect healing. No structural changes are observed after the fullerene
formation, suggesting that defect healing is a slow process in comparison with
the fullerene shell formation. The schemes of the revealed reactions of chain
atoms insertion into the fullerene shell just before its completion are
presented. The results of the performed simulations are summarized within the
paradigm of fullerene formation due to selforganization of the carbon system.Comment: 35 pages, 9 figure
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