5,727 research outputs found
Finding topological subgraphs is fixed-parameter tractable
We show that for every fixed undirected graph , there is a
time algorithm that tests, given a graph , if contains as a
topological subgraph (that is, a subdivision of is subgraph of ). This
shows that topological subgraph testing is fixed-parameter tractable, resolving
a longstanding open question of Downey and Fellows from 1992. As a corollary,
for every we obtain an time algorithm that tests if there is
an immersion of into a given graph . This answers another open question
raised by Downey and Fellows in 1992
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
The ammonia dimer (NH3)2 has been investigated using high--level ab initio
quantum chemistry methods and density functional theory (DFT). The structure
and energetics of important isomers is obtained to unprecedented accuracy
without resorting to experiment. The global minimum of eclipsed C_s symmetry is
characterized by a significantly bent hydrogen bond which deviates from
linearity by about 20 degrees. In addition, the so-called cyclic C_{2h}
structure is extremely close in energy on an overall flat potential energy
surface. It is demonstrated that none of the currently available (GGA,
meta--GGA, and hybrid) density functionals satisfactorily describe the
structure and relative energies of this nonlinear hydrogen bond. We present a
novel density functional, HCTH/407+, designed to describe this sort of hydrogen
bond quantitatively on the level of the dimer, contrary to e.g. the widely used
BLYP functional. This improved functional is employed in Car-Parrinello ab
initio molecular dynamics simulations of liquid ammonia to judge its
performance in describing the associated liquid. Both the HCTH/407+ and BLYP
functionals describe the properties of the liquid well as judged by analysis of
radial distribution functions, hydrogen bonding structure and dynamics,
translational diffusion, and orientational relaxation processes. It is
demonstrated that the solvation shell of the ammonia molecule in the liquid
phase is dominated by steric packing effects and not so much by directional
hydrogen bonding interactions. In addition, the propensity of ammonia molecules
to form bifurcated and multifurcated hydrogen bonds in the liquid phase is
found to be negligibly small.Comment: Journal of Chemical Physics, in press (305335JCP
Quasi-chemical study of Be(aq) speciation
Be(aq) hydrolysis can to lead to the formation of multi-beryllium
clusters, but the thermodynamics of this process has not been resolved
theoretically. We study the hydration state of an isolated Be ion using
both the quasi-chemical theory of solutions and ab initio molecular dynamics.
These studies confirm that Be(aq) is tetra-hydrated. The quasi-chemical
approach is then applied to then the deprotonation of Be(H_2O)_4^{2+}} to
give BeOH(H_2O)_3{}^{+}}. The calculated pK of 3.8 is in good agreement
with the experimentally suggested value around 3.5. The calculated energetics
for the formation of BeOHBe are then obtained in fair agreement with
experiments.Comment: 11 pages, 3 figure
Calibrating for Class Weights by Modeling Machine Learning
A much studied issue is the extent to which the confidence scores provided by
machine learning algorithms are calibrated to ground truth probabilities. Our
starting point is that calibration is seemingly incompatible with class
weighting, a technique often employed when one class is less common (class
imbalance) or with the hope of achieving some external objective
(cost-sensitive learning). We provide a model-based explanation for this
incompatibility and use our anthropomorphic model to generate a simple method
of recovering likelihoods from an algorithm that is miscalibrated due to class
weighting. We validate this approach in the binary pneumonia detection task of
Rajpurkar, Irvin, Zhu, et al. (2017).Comment: 14 pages, 4 figure
Exponential Time Complexity of the Permanent and the Tutte Polynomial
We show conditional lower bounds for well-studied #P-hard problems:
(a) The number of satisfying assignments of a 2-CNF formula with n variables
cannot be counted in time exp(o(n)), and the same is true for computing the
number of all independent sets in an n-vertex graph.
(b) The permanent of an n x n matrix with entries 0 and 1 cannot be computed
in time exp(o(n)).
(c) The Tutte polynomial of an n-vertex multigraph cannot be computed in time
exp(o(n)) at most evaluation points (x,y) in the case of multigraphs, and it
cannot be computed in time exp(o(n/polylog n)) in the case of simple graphs.
Our lower bounds are relative to (variants of) the Exponential Time
Hypothesis (ETH), which says that the satisfiability of n-variable 3-CNF
formulas cannot be decided in time exp(o(n)). We relax this hypothesis by
introducing its counting version #ETH, namely that the satisfying assignments
cannot be counted in time exp(o(n)). In order to use #ETH for our lower bounds,
we transfer the sparsification lemma for d-CNF formulas to the counting
setting
Process Mining for Advanced Service Analytics – From Process Efficiency to Customer Encounter and Experience
With the ongoing trend of servitization nurtured through digital technologies, the analysis of services as a starting point for improvement is gaining more and more importance. Service analytics has been defined as a concept to analyze the data generated during service execution to create value for providers and customers. To create more useful insights from the data, there is a continuous need for more advanced solutions for service analytics. One promising technology is process mining which has its origins in business process management. Our work provides insights into how process mining is currently used to analyze service processes and how it could be used along the service process. We find that process mining is increasingly applied for the analysis of the providers' internal operations, but more emphasis should be put on analyzing the customer interaction and experience
Ab initio molecular dynamics simulations of Aluminum solvation
The solvation of Al and its hydrolyzed species in water clusters has been
studied by means of ab initio molecular dynamics simulations. The hexa-hydrate
aluminum ion formed a stable complex in the finite temperature cluster
simulation of one aluminum ion and 16 waters. The average dipole moment of
strongly polarized hydrated water molecules in the first solvation shell of the
hexa-hydrate aluminum ion was found to be 5.02 Debye. The deprotonated
hexa-hydrate complex evolves into a tetra-coordinated aluminate ion with two
water molecules in the second solvation shell forming hydrogen bonds to the
hydroxyl groups in agreement with the observed coordination.Comment: 12 pages in Elsevier LaTeX, 5 figures in Postscript, 2 last figures
are in color, submitted to Chemical Physics Letter
Enumerating Homomorphisms
The homomorphism problem for relational structures is an abstract way of formulating constraint satisfaction problems (CSP) and various problems in database theory. The decision version of the homomorphism problem received a lot of attention in literature; in particular, the way the graph-theoretical structure of the variables and constraints influences the complexity of the problem is intensively studied. Here we study the problem of enumerating all the solutions with polynomial delay from a similar point of view. It turns out that the enumeration problem behaves very differently from the decision version. We give evidence that it is unlikely that a characterization result similar to the decision version can be obtained. Nevertheless, we show nontrivial cases where enumeration can be done with polynomial delay
Selective nanomechanics of aromatic versus aliphatic thiolates on gold surfaces
Thiolated gold nanointerfaces play a key role in numerous fields of science, technology, as well as modern medicine to coat, functionalize, and protect. Our computational study reveals that the mechanical vs thermal stabilities of aliphatic thiolates on gold surfaces are strikingly different from those of aromatic thiolates. The aliphatic thiolates feature, at the same time, a higher thermal desorption energy but a lower mechanical rupture force than thiophenolates. Our analysis discloses that this most counterintuitive property is due to different mechanochemical detachment mechanisms. Electronic structure analyses along the detachment pathways trace this back to the distinct electronic properties of the S─Au bond in stretched nanojunctions. The discoveries that it is a higher thermal stability that entails a lower mechanical stability and that mechanical loads generate different local nanostructures depending on the nature of the thiolate are highly relevant for the rational design of improved thiol-gold nanocontacts
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