495 research outputs found
Enhancing structure relaxations for first-principles codes: an approximate Hessian approach
We present a method for improving the speed of geometry relaxation by using a
harmonic approximation for the interaction potential between nearest neighbor
atoms to construct an initial Hessian estimate. The model is quite robust, and
yields approximately a 30% or better reduction in the number of calculations
compared to an optimized diagonal initialization. Convergence with this
initializer approaches the speed of a converged BFGS Hessian, therefore it is
close to the best that can be achieved. Hessian preconditioning is discussed,
and it is found that a compromise between an average condition number and a
narrow distribution in eigenvalues produces the best optimization.Comment: 9 pages, 3 figures, added references, expanded optimization sectio
PDFO: A Cross-Platform Package for Powell's Derivative-Free Optimization Solvers
The late Professor M. J. D. Powell devised five trust-region derivative-free
optimization methods, namely COBYLA, UOBYQA, NEWUOA, BOBYQA, and LINCOA. He
also carefully implemented them into publicly available solvers, which are
renowned for their robustness and efficiency. However, the solvers were
implemented in Fortran 77 and hence may not be easily accessible to some users.
We introduce the PDFO package, which provides user-friendly Python and MATLAB
interfaces to Powell's code. With PDFO, users of such languages can call
Powell's Fortran solvers easily without dealing with the Fortran code.
Moreover, PDFO includes bug fixes and improvements, which are particularly
important for handling problems that suffer from ill-conditioning or failures
of function evaluations. In addition to the PDFO package, we provide an
overview of Powell's methods, sketching them from a uniform perspective,
summarizing their main features, and highlighting the similarities and
interconnections among them. We also present experiments on PDFO to demonstrate
its stability under noise, tolerance of failures in function evaluations, and
potential in solving certain hyperparameter optimization problems
An Optimal Interpolation Set for Model-Based Derivative-Free Optimization Methods
This paper demonstrates the optimality of an interpolation set employed in
derivative-free trust-region methods. This set is optimal in the sense that it
minimizes the constant of well-poisedness in a ball centred at the starting
point. It is chosen as the default initial interpolation set by many
derivative-free trust-region methods based on underdetermined quadratic
interpolation, including NEWUOA, BOBYQA, LINCOA, and COBYQA. Our analysis
provides a theoretical justification for this choice
Development Of New Algorithms For Exploring The Potential Energy Landscape Of Chemical Reactions
The research presented in this dissertation is divided into 5 chapters. In Chapter 2, a method for reducing the number of coordinates required to accurately reproduce a known chemical reaction pathway by applying principal component analysis to a number of geometries along the pathway (expressed in either Cartesian coordinates or redundant internal coordinates) is described and applied to 9 example reactions. Chapter 3 introduces new methods for estimating the structure of and optimizing transition states by utilizing information about the atomic bonding in the reactants and products. These methods are then benchmarked against a standard transition state optimization approach utilizing a test set of 20 reactions, with energies computed at both semiempirical and density functional theory levels of theory. Chapter 4 is a collection of 3 new, alternative approaches (Flowchart Hessian updating, Scaled Rational Function Optimization, Quasi-Rotation coordinate propagation), to handling aspects of a typical Quasi-Newton minimization. These new approaches are then compared to their standard counterparts by optimizing a set of 20 molecules using either ab initio or density functional theory potential energy surfaces.
The final two chapters of this thesis focus on the development of a new path optimization framework, the Variational Reaction Coordinate (VRC) method. This framework seeks to improve upon the “chain of states” methods, which minimize the energy of a series of structures while using constraints, fictitious forces or reparameterization schemes to maintain the distribution of points along the path. In the VRC method, a functional representing the energy of the entire reaction is minimized by varying the expansion coefficients of a continuous function used to represent the reaction path. In Chapter 5, an algorithm is outlined along with the discussion and application of constraints and coupling terms that may be used to improve the efficiency and reliability of the method, with analytical test surfaces used to demonstrate the method’s performance. Chapter 6 focuses on the inclusion of redundant internal coordinates and methods for approximating the potential energy surface into the VRC framework, in order to reduce the per-iteration computational cost of the VRC method to something comparable to the “chain of states” approaches so that it may be practically applied to the study of reactions using high accuracy density functional theory and ab initio potential energy surfaces
PkANN - II. A non-linear matter power spectrum interpolator developed using artificial neural networks
In this paper we introduce PkANN, a freely available software package for
interpolating the non-linear matter power spectrum, constructed using
Artificial Neural Networks (ANNs). Previously, using Halofit to calculate
matter power spectrum, we demonstrated that ANNs can make extremely quick and
accurate predictions of the power spectrum. Now, using a suite of 6380 N-body
simulations spanning 580 cosmologies, we train ANNs to predict the power
spectrum over the cosmological parameter space spanning confidence
level (CL) around the concordance cosmology. When presented with a set of
cosmological parameters ( and redshift ), the trained ANN interpolates the power
spectrum for at sub-per cent accuracy for modes up to
. PkANN is faster than computationally expensive
N-body simulations, yet provides a worst-case error per cent fit to the
non-linear matter power spectrum deduced through N-body simulations. The
overall precision of PkANN is set by the accuracy of our N-body simulations, at
5 per cent level for cosmological models with eV for all
redshifts . For models with eV, predictions are
expected to be at 5 (10) per cent level for redshifts (). The
PkANN interpolator may be freely downloaded from
http://zuserver2.star.ucl.ac.uk/~fba/PkANNComment: 21 pages, 14 figures, 2 table
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