25 research outputs found
Real-Space Density Functional Theory on Graphical Processing Units: Computational Approach and Comparison to Gaussian Basis Set Methods
We
discuss the application of graphical processing units (GPUs)
to accelerate real-space density functional theory (DFT) calculations.
To make our implementation efficient, we have developed a scheme to
expose the data parallelism available in the DFT approach; this is
applied to the different procedures required for a real-space DFT
calculation. We present results for current-generation GPUs from AMD
and Nvidia, which show that our scheme, implemented in the free code
Octopus, can reach a sustained performance of up to 90 GFlops for
a single GPU, representing a significant speed-up when compared to
the CPU version of the code. Moreover, for some systems, our implementation
can outperform a GPU Gaussian basis set code, showing that the real-space
approach is a competitive alternative for DFT simulations on GPUs
Scalable High-Performance Algorithm for the Simulation of Exciton Dynamics. Application to the Light-Harvesting Complex II in the Presence of Resonant Vibrational Modes
The accurate simulation of excitonic
energy transfer in molecular
complexes with coupled electronic and vibrational degrees of freedom
is essential for comparing excitonic system parameters obtained from
ab initio methods with measured time-resolved spectra. Several exact
methods for computing the exciton dynamics within a density-matrix
formalism are known but are restricted to small systems with less
than 10 sites due to their computational complexity. To study the
excitonic energy transfer in larger systems, we adapt and extend the
exact hierarchical equation of motion (HEOM) method to various high-performance
many-core platforms using the Open Compute Language (OpenCL). For the light-harvesting
complex II (LHC II) found in spinach, the HEOM results deviate from
predictions of approximate theories and clarify the time scale of
the transfer process. We investigate the impact of resonantly coupled
vibrations on the relaxation and show that the transfer does not rely
on a fine-tuning of specific modes
Modeling Coherent Anti-Stokes Raman Scattering with Time-Dependent Density Functional Theory: Vacuum and Surface Enhancement.
We present the first density functional simulations of coherent anti-Stokes Raman scattering (CARS) and an analysis of the chemical effects upon binding to a metal surface. Spectra are obtained from first-principles electronic structure calculations and are compared with available experiments and previously available theoretical results following from HartreeāFock polarizability derivatives. A first approximation to the nonresonant portion of the CARS signal is also explored. We examine the silver pyridine cluster models of the surface chemical signal enhancement, previously introduced for surface-enhanced Raman scattering. Chemical resonant intensity enhancements of roughly 10<sup>2</sup> are found for several model clusters. The prospects of realizing further enhancement of CARS signal with metal surfaces is discussed in light of the predicted chemical enhancements
Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3ā<i>b</i>:2ā²3ā²ā<i>f</i>]thieno[3,2ā<i>b</i>]āthiophene Thin Films: The Importance of Hole Delocalization
Charge-transfer states in organic
semiconductors play crucial roles
in processes such as singlet fission and exciton dissociation at donor/acceptor
interfaces. Recently, a time-resolved spectroscopy study of dinaphthoĀ[2,3-<i>b</i>:2ā²3ā²-<i>f</i>]ĀthienoĀ[3,2-<i>b</i>]-thiophene (DNTT) thin films provided evidence for the
formation of mixed Frenkel and charge-transfer excitons after the
photoexcitation. Here, we investigate optical properties and excitation
dynamics of the DNTT thin films by combining ab initio calculations
and a stochastic SchroĢdinger equation. Our theory predicts
that the low-energy Frenkel exciton band consists of 8ā47%
CT character. The quantum dynamics simulations show coherent dynamics
of Frenkel and CT states in 50 fs after the optical excitation. We
demonstrate the role of charge delocalization and localization in
the mixing of CT states with Frenkel excitons as well as the role
of their decoherence
Complex Chemical Reaction Networks from Heuristics-Aided Quantum Chemistry
While
structures and reactivities of many small molecules can be
computed efficiently and accurately using quantum chemical methods,
heuristic approaches remain essential for modeling complex structures
and large-scale chemical systems. Here, we present a heuristics-aided
quantum chemical methodology applicable to complex chemical reaction
networks such as those arising in cell metabolism and prebiotic chemistry.
Chemical heuristics offer an expedient way of traversing high-dimensional
reactive potential energy surfaces and are combined here with quantum
chemical structure optimizations, which yield the structures and energies
of the reaction intermediates and products. Application of heuristics-aided
quantum chemical methodology to the formose reaction reproduces the
experimentally observed reaction products, major reaction pathways,
and autocatalytic cycles
Construction of the Fock Matrix on a Grid-Based Molecular Orbital Basis Using GPGPUs
We
present a GPGPU implementation of the construction of the Fock
matrix in the molecular orbital basis using the fully numerical, grid-based <i>bubbles</i> representation. For a test set of molecules containing
up to 90 electrons, the total HartreeāFock energies obtained
from reference GTO-based calculations are reproduced within 10<sup>ā4</sup> <i>E</i><sub>h</sub> to 10<sup>ā8</sup> <i>E</i><sub>h</sub> for most of the molecules studied.
Despite the very large number of arithmetic operations involved, the
high performance obtained made the calculations possible on a single
Nvidia Tesla K40 GPGPU card
Excitonics: A Set of Gates for Molecular Exciton Processing and Signaling
Regulating energy transfer pathways
through materials is a central goal of nanotechnology, as a greater
degree of control is crucial for developing sensing, spectroscopy,
microscopy, and computing applications. Such control necessitates
a toolbox of actuation methods that can direct energy transfer based
on user input. Here we introduce a proposal for a molecular exciton
gate, analogous to a traditional transistor, for regulating exciton
flow in chromophoric systems. The gate may be activated with an input
of light or an input flow of excitons. Our proposal relies on excitation
migration <i>via</i> the second excited singlet (S<sub>2</sub>) state of the gate molecule. It exhibits the following features,
only a subset of which are present in previous exciton switching schemes:
picosecond time scale actuation, amplification/gain behavior, and
a lack of molecular rearrangement. We demonstrate that the device
can be used to produce universal binary logic or amplification of
an exciton current, providing an excitonic platform with several potential
uses, including signal processing for microscopy and spectroscopy
methods that implement tunable exciton flux
Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates
Raman signals from molecules adsorbed on a noble metal surface are enhanced by many orders of magnitude due to the plasmon resonances of the substrate. Additionally, the enhanced spectra are modified compared to the spectra of neat molecules; many vibrational frequencies are shifted, and relative intensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attributed to the frequency dependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computationally using moleculeāmetal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate
Fast Delocalization Leads To Robust Long-Range Excitonic Transfer in a Large Quantum Chlorosome Model
Chlorosomes are efficient light-harvesting
antennas containing
up to hundreds of thousands of bacteriochlorophyll molecules. With
massively parallel computer hardware, we use a nonperturbative stochastic
SchroĢdinger equation, while including an atomistically derived
spectral density, to study excitonic energy transfer in a realistically
sized chlorosome model. We find that fast short-range delocalization
leads to robust long-range transfer due to the antennaeās concentric-roll
structure. Additionally, we discover anomalous behavior arising from
different initial conditions, and outline general considerations for
simulating excitonic systems on the nanometer to micrometer scale
Conformation and Electronic Population Transfer in Membrane-Supported Self-Assembled Porphyrin Dimers by 2D Fluorescence Spectroscopy
Two-dimensional fluorescence spectroscopy (2D FS) is
applied to
determine the conformation and femtosecond electronic population transfer
in a dimer of magnesium meso tetraphenylporphyrin. The dimers are
prepared by self-assembly of the monomer within the amphiphilic regions
of 1,2-distearoyl-<i>sn</i>-glycero-3-phosphocholine liposomes.
A theoretical framework to describe 2D FS experiments is presented,
and a direct comparison is made between the observables of this measurement
and those of 2D electronic spectroscopy (2D ES). The sensitivity of
the method to varying dimer conformation is explored. A global multivariable
fitting analysis of linear and 2D FS data indicates that the dimer
adopts a ābent T-shapedā conformation. Moreover, the
manifold of singly excited excitons undergoes rapid electronic dephasing
and downhill population transfer on the time scale of ā¼95 fs.
The open conformation of the dimer suggests that its self-assembly
is favored by an increase in entropy of the local membrane environment