71,291 research outputs found
Gradient type optimization methods for electronic structure calculations
The density functional theory (DFT) in electronic structure calculations can
be formulated as either a nonlinear eigenvalue or direct minimization problem.
The most widely used approach for solving the former is the so-called
self-consistent field (SCF) iteration. A common observation is that the
convergence of SCF is not clear theoretically while approaches with convergence
guarantee for solving the latter are often not competitive to SCF numerically.
In this paper, we study gradient type methods for solving the direct
minimization problem by constructing new iterations along the gradient on the
Stiefel manifold. Global convergence (i.e., convergence to a stationary point
from any initial solution) as well as local convergence rate follows from the
standard theory for optimization on manifold directly. A major computational
advantage is that the computation of linear eigenvalue problems is no longer
needed. The main costs of our approaches arise from the assembling of the total
energy functional and its gradient and the projection onto the manifold. These
tasks are cheaper than eigenvalue computation and they are often more suitable
for parallelization as long as the evaluation of the total energy functional
and its gradient is efficient. Numerical results show that they can outperform
SCF consistently on many practically large systems.Comment: 24 pages, 11 figures, 59 references, and 1 acknowledgement
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
A Computational Methodology to Screen Activities of Enzyme Variants
We present a fast computational method to efficiently screen enzyme activity.
In the presented method, the effect of mutations on the barrier height of an
enzyme-catalysed reaction can be computed within 24 hours on roughly 10
processors. The methodology is based on the PM6 and MOZYME methods as
implemented in MOPAC2009, and is tested on the first step of the amide
hydrolysis reaction catalyzed by Candida Antarctica lipase B (CalB) enzyme. The
barrier heights are estimated using adiabatic mapping and are shown to give
barrier heights to within 3kcal/mol of B3LYP/6-31G(d)//RHF/3-21G results for a
small model system. Relatively strict convergence criteria
(0.5kcal/(mol{\AA})), long NDDO cutoff distances within the MOZYME method
(15{\AA}) and single point evaluations using conventional PM6 are needed for
reliable results. The generation of mutant structure and subsequent setup of
the semiempirical calculations are automated so that the effect on barrier
heights can be estimated for hundreds of mutants in a matter of weeks using
high performance computing
Doping of graphene by a Au(111) substrate: Calculation strategy within the local density approximation and a semiempirical van der Waals approach
We have performed a density functional study of graphene adsorbed on Au(111)
surface using both a local density approximation and a semiempirical van der
Waals approach proposed by Grimme, known as the DFT-D2 method. Graphene
physisorbed on metal has the linear dispersion preserved in the band-structure,
but the Fermi level of the system is shifted with respect to the conical points
which results in a doping effect. We show that the type and amount of doping
depends not only on the choice of the exchange-correlation functional used in
the calculations, but also on the supercell geometry that models the physical
system. We analyzed how the factors such as the in-plane cell parameter and
interlayer spacing in gold influence the Fermi level shift and we found that
even a small variation in these parameters may cause a transition from p-type
to n-type doping. We have selected a reasonable set of model parameters and
obtained that graphene is either undoped or at most slightly p-type doped on
the clean Au(111) surface, which seems to be in line with experimental
findings. On the other hand, modifications of the substrate lattice may induce
larger doping up to 0.30-0.40 eV depending on the graphene-metal adsorption
distance. The sensitivity of the graphene-gold interface to the structural
parameters may allow to tune doping across the samples which could lead to
possible applications in graphene-based electronic devices. We believe that the
present remarks can be also useful for other studies based on the periodic DFT
Evolution of electronic and ionic structure of Mg-clusters with the growth cluster size
The optimized structure and electronic properties of neutral and singly
charged magnesium clusters have been investigated using ab initio theoretical
methods based on density-functional theory and systematic post-Hartree-Fock
many-body perturbation theory accounting for all electrons in the system. We
have systematically calculated the optimized geometries of neutral and singly
charged magnesium clusters consisting of up to 21 atoms, electronic shell
closures, binding energies per atom, ionization potentials and the gap between
the highest occupied and the lowest unoccupied molecular orbitals. We have
investigated the transition to the hcp structure and metallic evolution of the
magnesium clusters, as well as the stability of linear chains and rings of
magnesium atoms. The results obtained are compared with the available
experimental data and the results of other theoretical works.Comment: 30 pages, 10 figures, 3 table
New Approaches for ab initio Calculations of Molecules with Strong Electron Correlation
Reliable quantum chemical methods for the description of molecules with
dense-lying frontier orbitals are needed in the context of many chemical
compounds and reactions. Here, we review developments that led to our
newcomputational toolbo x which implements the quantum chemical density matrix
renormalization group in a second-generation algorithm. We present an overview
of the different components of this toolbox.Comment: 19 pages, 1 tabl
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