37 research outputs found
Systematic reduction of sign errors in many-body calculations of atoms and molecules
The self-healing diffusion Monte Carlo algorithm (SHDMC) [Phys. Rev. B {\bf
79}, 195117 (2009), {\it ibid.} {\bf 80}, 125110 (2009)] is shown to be an
accurate and robust method for calculating the ground state of atoms and
molecules. By direct comparison with accurate configuration interaction results
for the oxygen atom we show that SHDMC converges systematically towards the
ground-state wave function. We present results for the challenging N
molecule, where the binding energies obtained via both energy minimization and
SHDMC are near chemical accuracy (1 kcal/mol). Moreover, we demonstrate that
SHDMC is robust enough to find the nodal surface for systems at least as large
as C starting from random coefficients. SHDMC is a linear-scaling
method, in the degrees of freedom of the nodes, that systematically reduces the
fermion sign problem.Comment: Final version accepted in Physical Review Letters. The review history
(referees' comments and our replies) is included in the source
Self-consistent-field calculations using Chebyshev-filtered subspace iteration
Abstract The power of density functional theory is often limited by the high computational demand in solving an eigenvalue problem at each self-consistent-field (SCF) iteration. The method presented in this paper replaces the explicit eigenvalue calculations by an approximation of the wanted invariant subspace, obtained with the help of well-selected Chebyshev polynomial filters. In this approach, only the initial SCF iteration requires solving an eigenvalue problem, in order to provide a good initial subspace. In the remaining SCF iterations, no iterative eigensolvers are involved. Instead, Chebyshev polynomials are used to refine the subspace. The subspace iteration at each step is easily five to ten times faster than solving a corresponding eigenproblem by the most efficient eigen-algorithms. Moreover, the subspace iteration reaches self-consistency within roughly the same number of steps as an eigensolver-based approach. This results in a significantly faster SCF iteration
First-principles GW-BSE excitations in organic molecules
We present a first-principles method for the calculation of optical
excitations in nanosystems. The method is based on solving the Bethe-Salpeter
equation (BSE) for neutral excitations. The electron self-energy is evaluated
within the GW approximation, with dynamical screening effects described within
time-dependent density-functional theory in the adiabatic, local approximation.
This method is applied to two systems: the benzene molecule, CH, and
azobenzene, CHN. We give a description of the
photoisomerization process of azobenzene after an excitation,
which is consistent with multi-configuration calculations
Parallel Self-Consistent-Field Calculations via Chebyshev-Filtered Subspace Acceleration
Solving the Kohn-Sham eigenvalue problem constitutes the most computationally
expensive part in self-consistent density functional theory (DFT) calculations.
In a previous paper, we have proposed a nonlinear Chebyshev-filtered subspace
iteration method, which avoids computing explicit eigenvectors except at the
first SCF iteration. The method may be viewed as an approach to solve the
original nonlinear Kohn-Sham equation by a nonlinear subspace iteration
technique, without emphasizing the intermediate linearized Kohn-Sham eigenvalue
problem. It reaches self-consistency within a similar number of SCF iterations
as eigensolver-based approaches. However, replacing the standard
diagonalization at each SCF iteration by a Chebyshev subspace filtering step
results in a significant speedup over methods based on standard
diagonalization. Here, we discuss an approach for implementing this method in
multi-processor, parallel environment. Numerical results are presented to show
that the method enables to perform a class of highly challenging DFT
calculations that were not feasible before
Controlling the gap of fullerene microcrystals by applying pressure: the role of many-body effects
We studied theoretically the optical properties of C fullerene
microcrystals as a function of hydrostatic pressure with first-principles
many-body theories. Calculations of the electronic properties were done in the
GW approximation. We computed electronic excited states in the crystal by
diagonalizing the Bethe-Salpeter equation (BSE). Our results confirmed the
existence of bound excitons in the crystal. Both the electronic gap and optical
gap decrease continuously and non-linearly as pressure of up to 6 GPa is
applied. As a result, the absorption spectrum shows strong redshift. We also
obtained that "negative" pressure shows the opposite behavior: the gaps
increase and the optical spectrum shifts toward the blue end of the spectrum.
Negative pressure can be realized by adding cubane (CH) or other
molecules with similar size to the interstitials of the microcrystal. For the
moderate lattice distortions studied here, we found that the optical properties
of fullerene microcrystals with intercalated cubane are similar to the ones of
an expanded undoped microcrystal. Based on these findings, we propose doped C60
as active element in piezo-optical devices.Comment: Final version accepted by PRB. The review history is included in the
sourc
Optical excitations in organic molecules, clusters and defects studied by first-principles Green's function methods
Spectroscopic and optical properties of nanosystems and point defects are
discussed within the framework of Green's function methods. We use an approach
based on evaluating the self-energy in the so-called GW approximation and
solving the Bethe-Salpeter equation in the space of single-particle
transitions. Plasmon-pole models or numerical energy integration, which have
been used in most of the previous GW calculations, are not used. Fourier
transforms of the dielectric function are also avoided. This approach is
applied to benzene, naphthalene, passivated silicon clusters (containing more
than one hundred atoms), and the F center in LiCl. In the latter, excitonic
effects and the defect line are identified in the energy-resolved
dielectric function. We also compare optical spectra obtained by solving the
Bethe-Salpeter equation and by using time-dependent density functional theory
in the local, adiabatic approximation. From this comparison, we conclude that
both methods give similar predictions for optical excitations in benzene and
naphthalene, but they differ in the spectra of small silicon clusters. As
cluster size increases, both methods predict very low cross section for
photoabsorption in the optical and near ultra-violet ranges. For the larger
clusters, the computed cross section shows a slow increase as function of
photon frequency. Ionization potentials and electron affinities of molecules
and clusters are also calculated.Comment: 9 figures, 5 tables, to appear in Phys. Rev. B, 200
SiO2·p-TSA: a green catalyst for solvent-free tetrahydropyranylation of alcohols and thiols
A solvent-free procedure for tetrahydropyranylation of alcohols and thiols based on a simple grinding of the reagents in the presence of silica gel and catalytic amounts of p-TSA is described
Optical spectra and exchange-correlation effects in molecular crystals
We report first-principles GW-Bethe Salpeter Equation and Quantum Monte Carlo
calculations of the optical and electronic properties of molecular and
crystalline rubrene (CH). Many-body effects dominate the optical
spectrum and quasi-particle gap of molecular crystals. We interpret the
observed yellow-green photoluminescence in rubrene microcrystals as a result of
the formation of intermolecular, charge-transfer spin-singlet excitons. In
contrast, spin-triplet excitons are localized and intramolecular with a
predicted phosphorescence at the red end of the optical spectrum. We find that
the exchange energy plays a fundamental role in raising the energy of
intramolecular spin-singlet excitons above the intermolecular ones. Exciton
binding energies are predicted to be around 0.5 eV (spin singlet) to 1 eV (spin
triplet). The calculated electronic gap is 2.8 eV. The theoretical absorption
spectrum agrees very well with recent ellipsometry data.Comment: 4 pages, 4 figure