[Show abstract][Hide abstract] ABSTRACT: The correlation between transport properties across sub-nanometric metallic
gaps and the optical response of the system is a complex effect which is
determined by the fine atomic-scale details of the junction structure. As
experimental advances are progressively accessing transport and optical
characterization of smaller nanojunctions, a clear connection between the
structural, electronic and optical properties in these nanocavities is needed.
Using ab initio calculations, we present here a study of the simultaneous
evolution of the structure and the optical response of a plasmonic junction as
the particles forming the cavity, two Na$_{380}$ clusters, approach and
retract. Atomic reorganizations are responsible for a large hysteresis of the
plasmonic response of the system, that shows a jump-to-contact instability
during the approach process and the formation of an atom-sized neck across the
junction during retraction. Our calculations demonstrate that, due to the
quantization of the conductance in metal nanocontacts, atomic-scale
reconfigurations play a crucial role in determining the optical response of the
whole system. We observe abrupt changes in the intensities and spectral
positions of the dominating plasmon resonances, and find a one-to-one
correspondence between these jumps and those of the quantized transport as the
neck cross-section diminishes. These results point out to an unforeseen
connection between transport and optics at the atomic scale, which is at the
frontier of current optoelectronics and can drive new options in optical
engineering of signals driven by the motion and manipulation of single atoms.
[Show abstract][Hide abstract] ABSTRACT: A method is presented to compute the dielectric function for extended systems using linear response time-dependent density functional theory. Localized basis functions with finite support are used to expand both eigenstates and response functions. The electron-energy loss function is directly obtained by an iterative Krylov-subspace method. We apply our method to graphene and silicon and compare it to plane-wave based approaches. Finally, we compute electron-energy loss spectrum of C60 crystal to demonstrate the merits of the method for molecular crystals, where it will be most competitive.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 07/2015; 354. DOI:10.1016/j.nimb.2014.11.080 · 1.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Bethe-Salpeter equation (BSE) is currently the state of the art in the
description of neutral electron excitations in both solids and large finite
systems. It is capable of accurately treating charge-transfer excitations that
present difficulties for simpler approaches. We present a local basis set
formulation of the BSE for molecules where the optical spectrum is computed
with the iterative Haydock recursion scheme, leading to a low computational
complexity and memory footprint. Using a variant of the algorithm we can go
beyond the Tamm-Dancoff approximation (TDA). We rederive the recursion
relations for general matrix elements of a resolvent, show how they translate
into continued fractions, and study the convergence of the method with the
number of recursion coefficients and the role of different terminators. Due to
the locality of the basis functions the computational cost of each iteration
scales asymptotically as $O(N^3)$ with the number of atoms, while the number of
iterations is typically much lower than the size of the underlying
electron-hole basis. In practice we see that , even for systems with thousands
of orbitals, the runtime will be dominated by the $O(N^2)$ operation of
applying the Coulomb kernel in the atomic orbital representation
Physical Review B 05/2015; 92(7). DOI:10.1103/PhysRevB.92.075422 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electromagnetic field localization in nanoantennas is one of the leitmotivs that drives the development of plasmonics. The near-fields in these plasmonic nanoantennas are commonly addressed theoretically within classical frameworks that neglect atomic-scale features. This approach is often appropriate since the irregularities produced at the atomic scale are typically hidden in far-field optical spectroscopies. However, a variety of physical and chemical processes rely on the fine distribution of the local fields at this ultraconfined scale. We use time-dependent density functional theory and perform atomistic quantum mechanical calculations of the optical response of plasmonic nanoparticles, and their dimers, characterized by the presence of crystallographic planes, facets, vertices, and steps. Using sodium clusters as an example, we show that the atomistic details of the nanoparticles morphologies determine the presence of subnanometric near-field hot spots that are further enhanced by the action of the underlying nanometric plasmonic fields. This situation is analogue to a self-similar nanoantenna cascade effect, scaled down to atomic dimensions, and it provides new insights into the limits of field enhancement and confinement, with dramatic implications in the optical resolution of field-enhanced spectroscopies and microscopies.
[Show abstract][Hide abstract] ABSTRACT: Two self-consistent schemes involving Hedin's $GW$ approximation are studied
for a set of sixteen different atoms and small molecules. We compare results
from the fully self-consistent $GW$ approximation (SC$GW$) and the
quasi-particle self-consistent $GW$ approximation (QS$GW$) within the same
numerical framework. Core and valence electrons are treated on an equal footing
in all the steps of the calculation. We use basis sets of localized functions
to handle the space dependence of quantities and spectral functions to deal
with their frequency dependence. We compare SC$GW$ and QS$GW$ on a qualitative
level by comparing the computed densities of states (DOS). To judge their
relative merit on a quantitative level, we compare their vertical ionization
potentials (IPs) with those obtained from coupled-cluster calculations CCSD(T).
Our results are futher compared with "one-shot" $G_0W_0$ calculations starting
from Hartree-Fock solutions ($G_0W_0$-HF). Both self-consistent $GW$ approaches
behave quite similarly. Averaging over all the studied molecules, both methods
show only a small improvement (somewhat larger for SC$GW$) of the calculated
IPs with respect to $G_0W_0$-HF results. Interestingly, SC$GW$ and QS$GW$
calculations tend to deviate in opposite directions with respect to CCSD(T)
results. SC$GW$ systematically underestimates the IPs, while QS$GW$ tends to
overestimate them. $G_0W_0$-HF produces results which are surprisingly close to
QS$GW$ calculations both for the DOS and for the numerical values of the IPs.
[Show abstract][Hide abstract] ABSTRACT: Two self-consistent schemes involving Hedin's GW approximation are studied for a set of sixteen different atoms and small molecules. We compare results from the fully self-consistent GW approximation (SCGW) and the quasiparticle self-consistent GW approximation (QSGW) within the same numerical framework. Core and valence electrons are treated on an equal footing in all the steps of the calculation. We use basis sets of localized functions to handle the space dependence of quantities and spectral functions to deal with their frequency dependence. We compare SCGW and QSGW on a qualitative level by comparing the computed densities of states (DOS). To judge their relative merit on a quantitative level, we compare their vertical ionization potentials (IPs) with those obtained from coupled-cluster calculations CCSD(T). Our results are futher compared with "one-shot" G0W0 calculations starting from Hartree-Fock solutions (G0W0-HF). Both self-consistent GW approaches behave quite similarly. Averaging over all the studied molecules, both methods show only a small improvement (somewhat larger for SCGW) of the calculated IPs with respect to G0W0-HF results. Interestingly, SCGW and QSGW calculations tend to deviate in opposite directions with respect to CCSD(T) results. SCGW systematically underestimates the IPs, while QSGW tends to overestimate them. G0W0-HF produces results which are surprisingly close to QSGW calculations both for the DOS and for the numerical values of the IPs.
Physical Review B 03/2014; 89(15). DOI:10.1103/PhysRevB.89.155417 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules.
[Show abstract][Hide abstract] ABSTRACT: Many-body perturbation theory of bulk systems is often realized within
reciprocal space, using plane-wave (PW) basis sets. PW basis is
advantageous because of its elementary basis functions and simple
convergence control. However, the number of functions in PW basis grows
with third power of unit cell size, irrespective of actual number of
atoms present in the unit cell. Moreover, PW basis gives rise to full
matrices in tensor algebra due to space-filling nature of PW. An
alternative to PW would be usage of localized basis functions. In this
contribution, we show how a basis of dominant products (DP) can be used
to describe excitations in finite and bulk systems. We present
calculations of absorption spectra and electron-energy loss spectra
within time-dependent density functional theory, realized within DP
basis. The usage of localized functions and iterative techniques allow
to keep the complexity of the calculations rather low: the overall
number of operations grows with third power of number of atoms in the
unit cell.Moreover, we have recently shown that Hedin's GW calculations
can also be performed using DP basis with an order-N^3 scaling for
finite systems. We are currently extending this GW methodology to bulk
systems.
[Show abstract][Hide abstract] ABSTRACT: Hedin's GW approximation (GWA) is a well known method to study charged
excitations in electronic systems with a moderate computational cost
[1]. Already one-shot GWA delivers a considerable improvement if
compared with Green's functions from density-functional theory (DFT).
However, the one-shot results are dependent on the used starting point.
This unphysical dependence can be eliminated by iterating a GW
calculation to self-consistency. We implemented self-consistent GWA for
molecules [2], within our original framework of dominant products basis.
We use the DFT calculation by SIESTA code as starting point. The
framework allowed to calculate Green's functions on a fine frequency
mesh for such small molecules as benzene. We demonstrate the level of
independence on starting point achievable within pseudo-potential
framework, validating the implementation. Effects of the
self-consistency on the interacting Green's function will be discussed
along with different levels of self-consistency and mixing schemes.
Finally, we compare the self-consistency with so-called quasi-particle
self-consistent GW [3]. [0pt] [1] F.Aryasetiawan, O.Gunnarsson, Rep.
Prog. Phys. 61, 237 (1998).[0pt] [2] D.Foerster, P.Koval, D.Sanchez
Portal, J. Chem. Phys. 135, 074105 (2011).[0pt] [3] T.Kotani, M.van
Schilfgaarde, S.V.Falleev, Phys. Rev. B 76, 165106 (2007).
[Show abstract][Hide abstract] ABSTRACT: We describe an implementation of Hedin's GW approximation for molecules and clusters, the complexity of which scales as O(N(3)) with the number of atoms. Our method is guided by two strategies: (i) to respect the locality of the underlying electronic interactions and (ii) to avoid the singularities of Green's functions by manipulating, instead, their spectral functions using fast Fourier transform methods. To take into account the locality of the electronic interactions, we use a local basis of atomic orbitals and, also, a local basis in the space of their products. We further compress the screened Coulomb interaction into a space of lower dimensions for speed and to reduce memory requirements. The improved scaling of our method with respect to most of the published methodologies should facilitate GW calculations for large systems. Our implementation is intended as a step forward towards the goal of predicting, prior to their synthesis, the ionization energies and electron affinities of the large molecules that serve as constituents of organic semiconductors.
The Journal of Chemical Physics 08/2011; 135(7):074105. DOI:10.1063/1.3624731 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Organic electronics is a rapidly developing technology. Typically, the
molecules involved in organic electronics are made up of hundreds of atoms,
prohibiting a theoretical description by wavefunction-based ab-initio methods.
Density-functional and Green's function type of methods scale less steeply with
the number of atoms. Therefore, they provide a suitable framework for the
theory of such large systems.
In this contribution, we describe an implementation, for molecules, of
Hedin's GW approximation. The latter is the lowest order solution of a set of
coupled integral equations for electronic Green's and vertex functions that was
found by Lars Hedin half a century ago.
Our implementation of Hedin's GW approximation has two distinctive features:
i) it uses sets of localized functions to describe the spatial dependence of
correlation functions, and ii) it uses spectral functions to treat their
frequency dependence. Using these features, we were able to achieve a favorable
computational complexity of this approximation. In our implementation, the
number of operations grows as N^3 with the number of atoms N.
[Show abstract][Hide abstract] ABSTRACT: The use of the LCAO (Linear Combination of Atomic Orbitals) method for excited states involves products of orbitals that are known to be linearly dependent. We identify a basis in the space of orbital products that is local for orbitals of finite support and with a residual error that vanishes exponentially with its dimension. As an application of our previously reported technique we compute the Kohn--Sham density response function $\chi_{0}$ for a molecule consisting of $N$ atoms in $N^{2}N_{\omega}$ operations, with $N_{\omega}$ the number of frequency points. We test our construction of $\chi_{0}$ by computing molecular spectra directly from the equations of Petersilka--Gossmann--Gross in $N^{2}N_{\omega}$ operations rather than from Casida's equations which takes $N^{3}$ operations. We consider the good agreement with previously calculated molecular spectra as a validation of our construction of $\chi_{0}$. Ongoing work indicates that our method is well suited for the computation of the GW self-energy $\Sigma=\mathrm{i}GW$ and we expect it to be useful in the analysis of exitonic effects in molecules.
[Show abstract][Hide abstract] ABSTRACT: We describe a fast parallel iterative method for computing molecular
absorption spectra within TDDFT linear response and using the LCAO method. We
use a local basis of "dominant products" to parametrize the space of orbital
products that occur in the LCAO approach. In this basis, the dynamical
polarizability is computed iteratively within an appropriate Krylov subspace.
The iterative procedure uses a a matrix-free GMRES method to determine the
(interacting) density response. The resulting code is about one order of
magnitude faster than our previous full-matrix method. This acceleration makes
the speed of our TDDFT code comparable with codes based on Casida's equation.
The implementation of our method uses hybrid MPI and OpenMP parallelization in
which load balancing and memory access are optimized. To validate our approach
and to establish benchmarks, we compute spectra of large molecules on various
types of parallel machines.
The methods developed here are fairly general and we believe they will find
useful applications in molecular physics/chemistry, even for problems that are
beyond TDDFT, such as organic semiconductors, particularly in photovoltaics.
Journal of Chemical Theory and Computation 05/2010; 6(9). DOI:10.1021/ct100280x · 5.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We extend the LCAO (Linear Combination of Atomic Orbitals) method to excited states by constructing a particularly simple basis in the space of orbital products. The residual error of our procedure vanishes exponentially with the number of products and our procedure avoids auxiliary sets of fitting functions and their intrinsic ambiguities. As an application of our technique, we compute the Kohn--Sham density response function $\chi_{0}$ for a molecule consisting of $N$ atoms in $O(N^{2}N_{\omega })$ operations, with $N_{\omega }$ the number of frequency points. Our construction of $\chi_{0}$ allows us to compute molecular spectra directly from the equations of Petersilka--Gossmann--Gross in $O(N^{2}N_{\omega })$ operations rather than from Casida's equations which takes $O(N^{3})$ operations. Ongoing work indicates that our method is well suited to a computation of the GW self-energy $\Sigma=\mathrm{i}GW$ and we expect a similar situation for the Bethe--Salpeter equation.
[Show abstract][Hide abstract] ABSTRACT: We construct the Kohn-Sham density response function chi(0) in a previously described basis of the space of orbital products. The calculational complexity of our construction is O(N(2)N(omega)) for a molecule of N atoms and in a spectroscopic window of N(omega) frequency points. As a first application, we use chi(0) to calculate the molecular spectra from the Petersilka-Gossmann-Gross equation. With chi(0) as input, we obtain the correct spectra with an extra computational effort that grows also as O(N(2)N(omega)) and, therefore, less steeply in N than the O(N(3)) complexity of solving Casida's equations. Our construction should be useful for the study of excitons in molecular physics and in related areas where chi(0) is a crucial ingredient.
The Journal of Chemical Physics 08/2009; 131(4):044103. DOI:10.1063/1.3179755 · 2.95 Impact Factor