67 research outputs found
Getting excited: Challenges in quantum-classical studies of excitons in polymeric systems
A combination of classical molecular dynamics (MM/MD) and quantum chemical
calculations based on the density functional theory (DFT) was performed to
describe conformational properties of diphenylethyne (DPE), methylated-DPE and
poly para phenylene ethynylene (PPE). DFT calculations were employed to improve
and develop force field parameters for MM/MD simulations. Many-body Green's
functions theory within the GW approximation and the Bethe-Salpeter equation
were utilized to describe excited states of the systems. Reliability of the
excitation energies based on the MM/MD conformations was examined and compared
to the excitation energies from DFT conformations. The results show an overall
agreement between the optical excitations based on MM/MD conformations and DFT
conformations. This allows for calculation of excitation energies based on
MM/MD conformations
Morphology of Proliferating Epithelial Cellular Tissue
We investigate morphologies of proliferating cellular tissue using a newly
developed numerical simulation model for mechanical cell division. The model
reproduces structures of simple multi-cellular organisms via simple rules for
selective division and division plane orientation. The model is applied to a
bimodal mixture of stiff cells with a low growth potential and soft cells with
a high growth potential. In an even mixture, the soft cells develop into a
tissue matrix and the stiff cells into a dendrite-like network structure. For
soft cell inclusion in a stiff cellular matrix, the soft cells develop to a
fast growing tumour like structure that gradually evacuates the stiff cell
matrix. With increasing inter-cell friction, the tumour growth slows down and
parts of it is driven to self-inflicted cell death
A Kernel-based Machine Learning Approach to Computing Quasiparticle Energies within Many-Body Green's Functions Theory
We present a Kernel Ridge Regression (KRR) based supervised learning method
combined with Genetic Algorithms (GAs) for the calculation of quasiparticle
energies within Many-Body Green's Functions Theory. These energies representing
electronic excitations of a material are solutions to a set of non-linear
equations, containing the electron self-energy (SE) in the approximation.
Due to the frequency-dependence of this SE, standard approaches are
computationally expensive and may yield non-physical solutions, in particular
for larger systems. In our proposed model, we use KRR as a self-adaptive
surrogate model which reduces the number of explicit calculations of the SE.
Transforming the standard fixed-point problem of finding quasiparticle energies
into a global optimization problem with a suitably defined fitness function,
application of the GA yields uniquely the physically relevant solution. We
demonstrate the applicability of our method for a set of molecules from the
100 dataset, which are known to exhibit a particularly problematic
structure of the SE. Results of the KRR-GA model agree within less than 0.01 eV
with the reference standard implementation, while reducing the number of
required SE evaluations roughly by a factor of ten.Comment: 4 pages, 3 figures, conference: Machine Learning for Molecules
Workshop at NeurIPS 202
Jamming and force distribution in growing epithelial tissue
We investigate morphologies of proliferating cellular tissues using a newly
developed numerical simulation model for mechanical cell division and migration
in 2D. The model is applied to a bimodal mixture consisting of stiff cells with
a low growth potential and soft cells with a high growth potential; cancer
cells are typically considered to be softer than healthy cells. In an even
mixture, the soft cells develop into a tissue matrix and the stiff cells into a
dendrite-like network structure. When soft cells are placed inside a tissue
consisting of stiff cells (to model cancer growth), the soft cells develop to a
fast growing tumor-like structure that gradually evacuates the stiff cell
matrix. The model also demonstrates 1) how soft cells orient themselves in the
direction of the largest effective stiffness as predicted by the theory of
Bischofs and Schwarz (Proc. Natl. Acad. Sci U.S.A., 100, 9274--9279 (2003) and
2) that the orientation and force generation continue a few cell rows behind
the soft-stiff interface. With increasing inter-cell friction, tumor growth
slows down and cell death occurs. The contact force distribution between cells
is demonstrated to be highly sensitive to cell type mixtures and cell-cell
interactions, which indicates that local mechanical forces can be useful as a
regulator of tissue formation. The results shed new light on established
experimental data.Comment: arXiv admin note: text overlap with arXiv:1811.0757
Glassy dynamics from generalized mode-coupling theory: existence and uniqueness of solutions for hierarchically coupled integro-differential equations
Generalized mode-coupling theory (GMCT) is a first-principles-based and
systematically correctable framework to predict the complex relaxation dynamics
of glass-forming materials. The formal theory amounts to a hierarchy of
infinitely many coupled integro-differential equations, which may be
approximated using a suitable finite-order closure relation. Although previous
studies have suggested that finite-order GMCT leads to well-defined solutions,
and that the hierarchy converges as the closure level increases, no rigorous
and general result in this direction is known. Here we unambiguously establish
the existence and uniqueness of solutions to generic, schematic GMCT
hierarchies that are closed at arbitrary order. We consider two types of
commonly invoked closure approximations, namely mean-field and exponential
closures. We also distinguish explicitly between overdamped and underdamped
glassy dynamics, corresponding to hierarchies of first-order and second-order
integro-differential equations, respectively. We find that truncated GMCT
hierarchies closed under an exponential closure conform to previously developed
mathematical theories, such that the existence of a unique solution can be
readily inferred. Self-consistent mean-field closures, however, of which the
well-known standard-MCT closure approximation is a special case, warrant
additional arguments for mathematical rigour. We demonstrate that the existence
of a priori bounds on the solution is sufficient to also prove that unique
solutions exist for such self-consistent hierarchies. To complete our analysis,
we present simple arguments to show that these a priori bounds must exist,
motivated by the physical interpretation of the GMCT solutions as density
correlation functions. Overall, our work contributes to the theoretical
justification of GMCT for studies of the glass transition, placing GMCT on a
firmer mathematical footing.Comment: 7 pages Revised version that contains minor changes and, in Sec. V, a
revised argument. The former Sec. VI was suppressed and replaced by a more
stringent bound derived now in Sec.
Electronic Excitations in Complex Molecular Environments: Many-Body Green's Functions Theory in VOTCA-XTP
Many-body Green's functions theory within the GW approximation and the
Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP
software, aiming at the calculation of electronically excited states in complex
molecular environments. Based on Gaussian-type atomic orbitals and making use
of resolution of identify techniques, the code is designed specifically for
non-periodic systems. Application to the small molecule reference set
successfully validates the methodology and its implementation for a variety of
excitation types covering an energy range from 2-8 eV in single molecules.
Further, embedding each GW-BSE calculation into an atomistically resolved
surrounding, typically obtained from Molecular Dynamics, accounts for effects
originating from local fields and polarization. Using aqueous DNA as a
prototypical system, different levels of electrostatic coupling between the
regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid
to charge-transfer (CT) excitations in adenine base pairs. It is found that
their energy is extremely sensitive to the specific environment and to
polarization effects. The calculated redshift of the CT excitation energy
compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which
matches expectations from experimental data. Predicted lowest CT energies are
below that of a single nucleobase excitation, indicating the possibility of an
initial (fast) decay of such an UV excited state into a bi-nucleobase CT
exciton. The results show that VOTCA-XTP's GW-BSE/MM is a powerful tool to
study a wide range of types of electronic excitations in complex molecular
environments
Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent Organic Light-Emitting Diodes
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