36,667 research outputs found
RASCAL: calculation of graph similarity using maximum common edge subgraphs
A new graph similarity calculation procedure is introduced for comparing labeled graphs. Given a minimum similarity threshold, the procedure consists of an initial screening process to determine whether it is possible for the measure of similarity between the two graphs to exceed the minimum threshold, followed by a rigorous maximum common edge subgraph (MCES) detection algorithm to compute the exact degree and composition of similarity. The proposed MCES algorithm is based on a maximum clique formulation of the problem and is a significant improvement over other published algorithms. It presents new approaches to both lower and upper bounding as well as vertex selection
Automated Identification and Classification of Stereochemistry: Chirality and Double Bond Stereoisomerism
Stereoisomers have the same molecular formula and the same atom connectivity
and their existence can be related to the presence of different
three-dimensional arrangements. Stereoisomerism is of great importance in many
different fields since the molecular properties and biological effects of the
stereoisomers are often significantly different. Most drugs for example, are
often composed of a single stereoisomer of a compound, and while one of them
may have therapeutic effects on the body, another may be toxic. A challenging
task is the automatic detection of stereoisomers using line input
specifications such as SMILES or InChI since it requires information about
group theory (to distinguish stereoisomers using mathematical information about
its symmetry), topology and geometry of the molecule. There are several
software packages that include modules to handle stereochemistry, especially
the ones to name a chemical structure and/or view, edit and generate chemical
structure diagrams. However, there is a lack of software capable of
automatically analyzing a molecule represented as a graph and generate a
classification of the type of isomerism present in a given atom or bond.
Considering the importance of stereoisomerism when comparing chemical
structures, this report describes a computer program for analyzing and
processing steric information contained in a chemical structure represented as
a molecular graph and providing as output a binary classification of the isomer
type based on the recommended conventions. Due to the complexity of the
underlying issue, specification of stereochemical information is currently
limited to explicit stereochemistry and to the two most common types of
stereochemistry caused by asymmetry around carbon atoms: chiral atom and double
bond. A Webtool to automatically identify and classify stereochemistry is
available at http://nams.lasige.di.fc.ul.pt/tools.ph
Measurement cost of metric-aware variational quantum algorithms
Variational quantum algorithms are promising tools for near-term quantum
computers as their shallow circuits are robust to experimental imperfections.
Their practical applicability, however, strongly depends on how many times
their circuits need to be executed for sufficiently reducing shot-noise. We
consider metric-aware quantum algorithms: variational algorithms that use a
quantum computer to efficiently estimate both a matrix and a vector object. For
example, the recently introduced quantum natural gradient approach uses the
quantum Fisher information matrix as a metric tensor to correct the gradient
vector for the co-dependence of the circuit parameters. We rigorously
characterise and upper bound the number of measurements required to determine
an iteration step to a fixed precision, and propose a general approach for
optimally distributing samples between matrix and vector entries. Finally, we
establish that the number of circuit repetitions needed for estimating the
quantum Fisher information matrix is asymptotically negligible for an
increasing number of iterations and qubits.Comment: 17 pages, 3 figure
Machine learning reveals orbital interaction in crystalline materials
We propose a novel representation of crystalline materials named
orbital-field matrix (OFM) based on the distribution of valence shell
electrons. We demonstrate that this new representation can be highly useful in
mining material data. Our experiment shows that the formation energies of
crystalline materials, the atomization energies of molecular materials, and the
local magnetic moments of the constituent atoms in transition metal--rare-earth
metal bimetal alloys can be predicted with high accuracy using the OFM.
Knowledge regarding the role of coordination numbers of transition-metal and
rare-earth metal elements in determining the local magnetic moment of
transition metal sites can be acquired directly from decision tree regression
analyses using the OFM.Comment: 10 page
Tricritical scaling at the N_t=6 chiral phase transition for 2 flavour lattice QCD with staggered quarks
We have simulated lattice QCD directly in the chiral limit of zero quark mass
by adding an additional, irrelevant 4-fermion interaction to the standard
action. Using lattices having temporal extent of six and spatial extents of
twelve and eighteen, we find that the theory with 2 massless staggered quark
flavors has a second order finite temperature phase transition. The critical
exponents , and are measured and favour tricritical
behaviour over that expected by universality arguments. The pion screening mass
is consistent with zero below the transition, but is degenerate with the
nonzero mass above the transition, indicating the restoration of
chiral symmetry.Comment: 12 pages, 3 postscript figures Letter has been rewritten to reflect a
reinterpretation of our results as indicating tricritical critical indices
rather than those predicted by standard universality arguments. Some
additional analysis which supports this interpretation is included. The
comparison of our methods with those of others has been expande
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