18 research outputs found
MADNESS: A Multiresolution, Adaptive Numerical Environment for Scientific Simulation
MADNESS (multiresolution adaptive numerical environment for scientific
simulation) is a high-level software environment for solving integral and
differential equations in many dimensions that uses adaptive and fast harmonic
analysis methods with guaranteed precision based on multiresolution analysis
and separated representations. Underpinning the numerical capabilities is a
powerful petascale parallel programming environment that aims to increase both
programmer productivity and code scalability. This paper describes the features
and capabilities of MADNESS and briefly discusses some current applications in
chemistry and several areas of physics
An efficient numerical quadrature for the calculation of the potential energy of wavefunctions expressed in the Daubechies wavelet basis
An efficient numerical quadrature is proposed for the approximate calculation
of the potential energy in the context of pseudo potential electronic structure
calculations with Daubechies wavelet and scaling function basis sets. Our
quadrature is also applicable in the case of adaptive spatial resolution. Our
theoretical error estimates are confirmed by numerical test calculations of the
ground state energy and wave function of the harmonic oscillator in one
dimension with and without adaptive resolution. As a byproduct we derive a
filter, which, upon application on the scaling function coefficients of a
smooth function, renders the approximate grid values of this function. This
also allows for a fast calculation of the charge density from the wave
function.Comment: 35 pages, 9 figures. Submitted to: Journal of Computational Physic
Wavelet-Based Linear-Response Time-Dependent Density-Functional Theory
Linear-response time-dependent (TD) density-functional theory (DFT) has been
implemented in the pseudopotential wavelet-based electronic structure program
BigDFT and results are compared against those obtained with the all-electron
Gaussian-type orbital program deMon2k for the calculation of electronic
absorption spectra of N2 using the TD local density approximation (LDA). The
two programs give comparable excitation energies and absorption spectra once
suitably extensive basis sets are used. Convergence of LDA density orbitals and
orbital energies to the basis-set limit is significantly faster for BigDFT than
for deMon2k. However the number of virtual orbitals used in TD-DFT calculations
is a parameter in BigDFT, while all virtual orbitals are included in TD-DFT
calculations in deMon2k. As a reality check, we report the x-ray crystal
structure and the measured and calculated absorption spectrum (excitation
energies and oscillator strengths) of the small organic molecule
N-cyclohexyl-2-(4-methoxyphenyl)imidazo[1,2-a]pyridin-3-amine
Kinetic energy-free Hartree–Fock equations: an integral formulation
We have implemented a self-consistent feld solver for Hartree–Fock calculations,
by making use of Multiwavelets and Multiresolution Analysis. We show how such a
solver is inherently a preconditioned steepest descent method and therefore a good
starting point for rapid convergence. A distinctive feature of our implementation is
the absence of any reference to the kinetic energy operator. This is desirable when
Multiwavelets are employed, because diferential operators such as the Laplacian in
the kinetic energy are challenging to represent correctly. The theoretical framework
is described in detail and the implemented algorithm is both presented in the paper
and made available as a Python notebook. Two simple examples are presented, highlighting the main features of our implementation: arbitrary predefned precision,
rapid and robust convergence, absence of the kinetic energy operator
Uncovering Structure-Property Relationships in Push-Pull Chromophores: A Promising Route to Large Hyperpolarizability and Two-Photon Absorption
In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials
Approximating a Wavefunction as an Unconstrained Sum of Slater Determinants
The wavefunction for the multiparticle Schr\"odinger equation is a function
of many variables and satisfies an antisymmetry condition, so it is natural to
approximate it as a sum of Slater determinants. Many current methods do so, but
they impose additional structural constraints on the determinants, such as
orthogonality between orbitals or an excitation pattern. We present a method
without any such constraints, by which we hope to obtain much more efficient
expansions, and insight into the inherent structure of the wavefunction. We use
an integral formulation of the problem, a Green's function iteration, and a
fitting procedure based on the computational paradigm of separated
representations. The core procedure is the construction and solution of a
matrix-integral system derived from antisymmetric inner products involving the
potential operators. We show how to construct and solve this system with
computational complexity competitive with current methods.Comment: 30 page