62 research outputs found
Computational Design of Nanoclusters by Property-Based Genetic Algorithms: Tuning the Electronic Properties of (TiO) Clusters
In order to design clusters with desired properties, we have implemented a
suite of genetic algorithms tailored to optimize for low total energy, high
vertical electron affinity (VEA), and low vertical ionization potential (VIP).
Applied to (TiO) clusters, the property-based optimization reveals the
underlying structure-property relations and the structural features that may
serve as active sites for catalysis. High VEA and low VIP are correlated with
the presence of several dangling-O atoms and their proximity, respectively. We
show that the electronic properties of (TiO) up to n=20 correlate more
strongly with the presence of these structural features than with size.Comment: 4 figs, 5 page
Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride
The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is
investigated via a van der Waals corrected density functional theory approach.
It is found that the main role of the van der Waals forces is to "anchor" the
layers at a fixed distance, whereas the electrostatic forces dictate the
optimal stacking mode and the interlayer sliding energy. A nearly free-sliding
path is identified, along which bandgap modulations of ~0.6 eV are obtained. We
propose a simple geometrical model that quantifies the registry matching
between the layers and captures the essence of the corrugated h-BN interlayer
energy landscape. The simplicity of this phenomenological model opens the way
to the modeling of complex layered structures, such as carbon and boron nitride
nanotubes.Comment: 4 Pages, 3 Figure
Electrodynamic Response and Stability of Molecular Crystals
We show that electrodynamic dipolar interactions, responsible for long-range
fluctuations in matter, play a significant role in the stability of molecular
crystals. Density functional theory calculations with van der Waals
interactions determined from a semilocal "atom-in-a-molecule" model result in a
large overestimation of the dielectric constants and sublimation enthalpies for
polyacene crystals from naphthalene to pentacene, whereas an accurate treatment
of non-local electrodynamic response leads to an agreement with the measured
values for both quantities. Our findings suggest that collective response
effects play a substantial role not only for optical excitations, but also for
cohesive properties of non-covalently bound molecular crystals
Electronic Structure of Copper Phthalocyanine: a Comparative Density Functional Theory Study
We present a systematic density functional theory study of the electronic
structure of copper phthalocyanine (CuPc), using several different (semi)-local
and hybrid functionals, and compare the results to experimental photoemission
data. We show that semi-local functionals fail qualitatively for CuPc,
primarily because of under-binding of localized orbitals due to
self-interaction errors. We discuss an appropriate choice of functional for
studies of CuPc/metal interfaces and suggest the Heyd-Scuseria-Ernzerhof
screened hybrid functional as a suitable compromise functional.Comment: 18 pages, 4 Figure, 1 Tabl
A Benchmark of GW Methods for Azabenzenes: Is the GW Approximation Good Enough?
Many-body perturbation theory in the GW approximation is a useful method for
describing electronic properties associated with charged excitations. A
hierarchy of GW methods exists, starting from non-self-consistent G0W0, through
partial self-consistency in the eigenvalues (ev-scGW) and in the Green function
(scGW0), to fully self-consistent GW (scGW). Here, we assess the performance of
these methods for benzene, pyridine, and the diazines. The quasiparticle
spectra are compared to photoemission spectroscopy (PES) experiments with
respect to all measured particle removal energies and the ordering of the
frontier orbitals. We find that the accuracy of the calculated spectra does not
match the expectations based on their level of self-consistency. In particular,
for certain starting points G0W0 and scGW0 provide spectra in better agreement
with the PES than scGW
GAtor: A First Principles Genetic Algorithm for Molecular Crystal Structure Prediction
We present the implementation of GAtor, a massively parallel, first
principles genetic algorithm (GA) for molecular crystal structure prediction.
GAtor is written in Python and currently interfaces with the FHI-aims code to
perform local optimizations and energy evaluations using dispersion-inclusive
density functional theory (DFT). GAtor offers a variety of fitness evaluation,
selection, crossover, and mutation schemes. Breeding operators designed
specifically for molecular crystals provide a balance between exploration and
exploitation. Evolutionary niching is implemented in GAtor by using machine
learning to cluster the dynamically updated population by structural similarity
and then employing a cluster-based fitness function. Evolutionary niching
promotes uniform sampling of the potential energy surface by evolving several
sub-populations, which helps overcome initial pool biases and selection biases
(genetic drift). The various settings offered by GAtor increase the likelihood
of locating numerous low-energy minima, including those located in
disconnected, hard to reach regions of the potential energy landscape. The best
structures generated are re-relaxed and re-ranked using a hierarchy of
increasingly accurate DFT functionals and dispersion methods. GAtor is applied
to a chemically diverse set of four past blind test targets, characterized by
different types of intermolecular interactions. The experimentally observed
structures and other low-energy structures are found for all four targets. In
particular, for Target II, 5-cyano-3-hydroxythiophene, the top ranked putative
crystal structure is a =2 structure with P symmetry and a
scaffold packing motif, which has not been reported previously
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