305 research outputs found
Mechanism Deduction from Noisy Chemical Reaction Networks
We introduce KiNetX, a fully automated meta-algorithm for the kinetic
analysis of complex chemical reaction networks derived from semi-accurate but
efficient electronic structure calculations. It is designed to (i) accelerate
the automated exploration of such networks, and (ii) cope with model-inherent
errors in electronic structure calculations on elementary reaction steps. We
developed and implemented KiNetX to possess three features. First, KiNetX
evaluates the kinetic relevance of every species in a (yet incomplete) reaction
network to confine the search for new elementary reaction steps only to those
species that are considered possibly relevant. Second, KiNetX identifies and
eliminates all kinetically irrelevant species and elementary reactions to
reduce a complex network graph to a comprehensible mechanism. Third, KiNetX
estimates the sensitivity of species concentrations toward changes in
individual rate constants (derived from relative free energies), which allows
us to systematically select the most efficient electronic structure model for
each elementary reaction given a predefined accuracy. The novelty of KiNetX
consists in the rigorous propagation of correlated free-energy uncertainty
through all steps of our kinetic analyis. To examine the performance of KiNetX,
we developed AutoNetGen. It semirandomly generates chemistry-mimicking reaction
networks by encoding chemical logic into their underlying graph structure.
AutoNetGen allows us to consider a vast number of distinct chemistry-like
scenarios and, hence, to discuss assess the importance of rigorous uncertainty
propagation in a statistical context. Our results reveal that KiNetX reliably
supports the deduction of product ratios, dominant reaction pathways, and
possibly other network properties from semi-accurate electronic structure data.Comment: 36 pages, 4 figures, 2 table
Gradient-Driven Molecule Construction: An Inverse Approach Applied to the Design of Small-Molecule Fixating Catalysts
Rational design of molecules and materials usually requires extensive
screening of molecular structures for the desired property. The inverse
approach to deduce a structure for a predefined property would be highly
desirable, but is, unfortunately, not well-defined. However, feasible
strategies for such an inverse design process may be successfully developed for
specific purposes. We discuss options for calculating 'jacket' potentials that
fulfill a predefined target requirement - a concept that we recently introduced
[T. Weymuth, M. Reiher, MRS Proceediungs, 2013, 1524,
DOI:10.1557/opl.2012.1764]. We consider the case of small-molecule activating
transition metal catalysts. As a target requirement we choose the vanishing
geometry gradients on all atoms of a subsystem consisting of a metal center
binding the small molecule to be activated. The jacket potential can be
represented within a full quantum model or by a sequence of approximations of
which a field of electrostatic point charges is the simplest. In a second step,
the jacket potential needs to be replaced by a chemically viable chelate-ligand
structure for which the geometry gradients on all of its atoms are also
required to vanish. In order to analyze the feasibility of this approach, we
dissect a known dinitrogen-fixating catalyst to study possible design
strategies that must eventually produce the known catalyst.Comment: 40 pages, 6 tables, 5 figure
Inverse Quantum Chemistry: Concepts and Strategies for Rational Compound Design
The rational design of molecules and materials is becoming more and more
important. With the advent of powerful computer systems and sophisticated
algorithms, quantum chemistry plays an important role in rational design. While
traditional quantum chemical approaches predict the properties of a predefined
molecular structure, the goal of inverse quantum chemistry is to find a
structure featuring one or more desired properties. Herein, we review inverse
quantum chemical approaches proposed so far and discuss their advantages as
well as their weaknesses.Comment: 43 pages, 5 figure
Molecular propensity as a driver for explorative reactivity studies
Quantum chemical studies of reactivity involve calculations on a large number
of molecular structures and comparison of their energies. Already the set-up of
these calculations limits the scope of the results that one will obtain,
because several system-specific variables such as the charge and spin need to
be set prior to the calculation. For a reliable exploration of reaction
mechanisms, a considerable number of calculations with varying global
parameters must be taken into account, or important facts about the reactivity
of the system under consideration can go undetected. For example, one could
miss crossings of potential energy surfaces for different spin states or might
not note that a molecule is prone to oxidation. Here, we introduce the concept
of molecular propensity to account for the predisposition of a molecular system
to react across different electronic states in certain nuclear configurations.
Within our real-time quantum chemistry framework, we developed an algorithm
that allows us to be alerted to such a propensity of a system under
consideration.Comment: 10 pages, 7 figure
Automated Selection of Active Orbital Spaces
One of the key challenges of quantum-chemical multi-configuration methods is
the necessity to manually select orbitals for the active space. This selection
requires both expertise and experience and can therefore impose severe
limitations on the applicability of this most general class of ab initio
methods. A poor choice of the active orbital space may yield even qualitatively
wrong results. This is obviously a severe problem, especially for wave function
methods that are designed to be systematically improvable. Here, we show how
the iterative nature of the density matrix renormalization group combined with
its capability to include up to about one hundred orbitals in the active space
can be exploited for a systematic assessment and selection of active orbitals.
These benefits allow us to implement an automated approach for active orbital
space selection, which can turn multi-configuration models into black box
approaches.Comment: 29 pages, 10 figures, 5 table
Measuring Multi-Configurational Character by Orbital Entanglement
One of the most critical tasks at the very beginning of a quantum chemical
investigation is the choice of either a multi- or single-configurational
method. Naturally, many proposals exist to define a suitable diagnostic of the
multi-configurational character for various types of wave functions in order to
assist this crucial decision. Here, we present a new orbital-entanglement based
multi-configurational diagnostic termed . The correspondence of
orbital entanglement and static (or nondynamic) electron correlation permits
the definition of such a diagnostic. We chose our diagnostic to meet important
requirements such as well-defined limits for pure single-configurational and
multi-configurational wave functions. The diagnostic can be
evaluated from a partially converged, but qualitatively correct, and therefore
inexpensive density matrix renormalization group wave function as in our
recently presented automated active orbital selection protocol. Its robustness
and the fact that it can be evaluated at low cost make this diagnostic a
practical tool for routine applications.Comment: 8 pages, 2 figure, 3 table
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