175 research outputs found
Self-Consistent Green Function Embedding for Advanced Electronic Structure Methods Based on a Dynamical Mean-Field Concept
We present an embedding scheme for periodic systems that facilitates the
treatment of the physically important part (here the unit cell) with advanced
electronic-structure methods, that are computationally too expensive for
periodic systems. The rest of the periodic system is treated with
computationally less demanding approaches, e.g., Kohn-Sham density-functional
theory, in a self- consistent manner. Our scheme is based on the concept of
dynamical mean-field theory (DMFT) formulated in terms of Green functions. In
contrast to the original DMFT formulation for correlated model Hamiltonians, we
here consider the unit cell as local embedded cluster in a first-principles
way, that includes all electronic degrees of freedom. Our real-space dynamical
mean-field embedding (RDMFE) scheme features two nested Dyson equations, one
for the embedded cluster and another for the periodic surrounding. The total
energy is computed from the resulting Green functions. The performance of our
scheme is demonstrated by treating the embedded region with hybrid functionals
and many-body perturbation theory in the GW approach for simple bulk systems.
The total energy and the density of states converge rapidly with respect to the
computational parameters and approach their bulk limit with increasing cluster
(i.e., unit cell) size
Thermodynamic equilibrium conditions of graphene films on SiC
First-principles surface phase diagrams reveal that epitaxial monolayer
graphene films on the Si side of 3C-SiC(111) can exist as thermodynamically
stable phases in a narrow range of experimentally controllable conditions,
defining a path to the highest-quality graphene films. Our calculations are
based on a van der Waals corrected density functional. The full, experimentally
observed (6 sqrt(3)x 6 sqrt(3))-R30 supercells for zero- to trilayer graphene
are essential to describe the correct interface geometries and the relative
stability of surface phases and possible defects
Exact-exchange based quasiparticle energy calculations for the band gap, effective masses and deformation potentials of ScN
The band gaps, longitudinal and transverse effective masses, and deformation
potentials of ScN in the rock-salt structure have been calculated employing
G0W0-quasiparticle calculations using exact-exchange Kohn-Sham density
functional theory one-particle wavefunctions and energies as input. Our
quasiparticle gaps support recent experimental observations that ScN has a much
lower indirect band gap than previously thought. The results are analyzed in
terms of the influence of different approximations for exchange and correlation
taken in the computational approach on the electronic structure of ScN.Comment: Comments: 9 pages including 5 figures; related publications can be
found at http://www.fhi-berlin.mpg.de/th/th.htm
Renormalized Second-order Perturbation Theory for The Electron Correlation Energy: Concept, Implementation, and Benchmarks
We present a renormalized second-order perturbation theory (rPT2), based on a
Kohn-Sham (KS) reference state, for the electron correlation energy that
includes the random-phase approximation (RPA), second-order screened exchange
(SOSEX), and renormalized single excitations (rSE). These three terms all
involve a summation of certain types of diagrams to infinite order, and can be
viewed as "renormalization" of the 2nd-order direct, exchange, and single
excitation (SE) terms of Rayleigh-Schr\"odinger perturbation theory based on an
KS reference. In this work we establish the concept of rPT2 and present the
numerical details of our SOSEX and rSE implementations. A preliminary version
of rPT2, in which the renormalized SE (rSE) contribution was treated
approximately, has already been benchmarked for molecular atomization energies
and chemical reaction barrier heights and shows a well balanced performance
[Paier et al, New J. Phys. 14, 043002 (2012)]. In this work, we present a
refined version of rPT2, in which we evaluate the rSE series of diagrams
rigorously. We then extend the benchmark studies to non-covalent interactions,
including the rare-gas dimers, and the S22 and S66 test sets. Despite some
remaining shortcomings, we conclude that rPT2 gives an overall satisfactory
performance across different chemical environments, and is a promising step
towards a generally applicable electronic structure approach.Comment: 16 pages, 11 figure
Density-functional Theory for f electron Systems: the {\alpha}-{\gamma} Phase Transition in Cerium
The isostructural {\alpha}-{\gamma} phase transition in cerium is analyzed
using density-functional theory with different exchange-correlation
functionals, in particular the PBE0 hybrid functional and the exact- exchange
plus correlation in the random-phase approximation [(EX+cRPA)@PBE0] approach.
We show that the Hartree-Fock exchange part of the hybrid functional actuates
two distinct solutions at zero temperature that can be associated with the
{\alpha} and {\gamma} phases of cerium. However, despite the relatively good
structural and magnetic properties, PBE0 predicts the {\gamma} phase to be the
stable phase at ambient pressure and zero temperature, in contradiction with
low temperature experiments. EX+cRPA reverses the energetic ordering, which
emphasizes the importance of correlation for rare- earth systems
Unified description of ground and excited states of finite systems: the self-consistent GW approach
GW calculations with fully self-consistent G and W -- based on the iterative
solution of the Dyson equation -- provide an approach for consistently
describing ground and excited states on the same quantum mechanical level. We
show that for the systems considered here self-consistent GW reaches the same
final Green function regardless of the initial reference state.
Self-consistency systematically improves ionization energies and total energies
of closed shell systems compared to G_0W_0 based on Hartree-Fock and
(semi)local density-functional theory. These improvements also translate to the
electron density as exemplified by an improved description of dipole moments
and permit us to assess the quality of ground state properties such as bond
lengths and vibrational frequencies.Comment: 5 pages, 4 figures, supplemental materia
Interface dipoles of organic molecules on Ag(111) in hybrid density-functional theory
We investigate the molecular acceptors 3,4,9,10-perylene-tetracarboxylic acid
dianhydride (PTCDA), 2,3,5,6-tetra uoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ), and 4,5,9,10-pyrenetetraone (PYTON) on Ag(111) using
densityfunctional theory. For two groups of the HSE(\alpha, \omega) family of
exchange-correlation functionals (\omega = 0 and \omega = 0.2\AA) we study the
isolated components as well as the combined systems as a function of the amount
of exact-exchange (\alpha). We find that hybrid functionals favour electron
transfer to the adsorbate. Comparing to experimental work-function data, we
report for (\alpha) ca. 0.25 a notable but small improvement over (semi)local
functionals for the interface dipole. Although Kohn-Sham eigenvalues are only
approximate representations of ionization energies, incidentally, at this value
also the density of states agrees well with the photoelectron spectra. However,
increasing (\alpha) to values for which the energy of the lowest unoccupied
molecular orbital matches the experimental electron affinity in the gas phase
worsens both the interface dipole and the density of states. Our results imply
that semi-local DFT calculations may often be adequate for conjugated organic
molecules on metal surfaces and that the much more computationally demanding
hybrid functionals yield only small improvements.Comment: submitted to New Journal of Physics (2013). More information can be
found at
http://th.fhi-berlin.mpg.de/site/index.php?n=Publications.Publication
First-Principles Description of Charge Transfer in Donor-Acceptor Compounds from Self-Consistent Many-Body Perturbation Theory
We investigate charge transfer in prototypical molecular donor-acceptor
compounds using hybrid density functional theory (DFT) and the GW approximation
at the perturbative level (G0W0) and at full self-consistency (sc-GW). For the
systems considered here, no charge transfer should be expected at large
intermolecular separation according to photoemission experiment and accurate
quantum-chemistry calculations. The capability of hybrid exchange-correlation
functionals of reproducing this feature depends critically on the fraction of
exact exchange , as for small values of spurious fractional
charge transfer is observed between the donor and the acceptor. G0W0 based on
hybrid DFT yields the correct alignment of the frontier orbitals for all values
of . However, G0W0 has no capacity to alter the ground-state properties
of the system, because of its perturbative nature. The electron density in
donor-acceptor compounds thus remains incorrect for small values. In
sc-GW, where the Green's function is obtained from the iterative solution of
the Dyson equation, the electron density is updated and reflects the correct
description of the level alignment at the GW level, demonstrating the
importance of self-consistent many-body approaches for the description of
ground- and excited-state properties in donor-acceptor systems.Comment: 8 pages, 7 figure
Length Dependence of Ionization Potentials of Trans-Acetylenes: Internally-Consistent DFT/GW Approach
We follow the evolution of the Ionization Potential (IP) for the paradigmatic
quasi-one-dimensional trans-acetylene family of conjugated molecules, from
short to long oligomers and to the infinite polymer trans-poly-acetylene (TPA).
Our results for short oligomers are very close to experimental available data.
We find that the IP varies with oligomer length and converges to the given
value for TPA with a smooth, coupled inverse-length-exponential behavior. Our
prediction is based on an "internally-consistent" scheme to adjust the exchange
mixing parameter of the PBEh hybrid density functional, so as to
obtain a description of the electronic structure consistent with the
quasiparticle approximation for the IP. This is achieved by demanding that the
corresponding quasiparticle correction, in the GW@PBEh approximation, vanishes
for the IP when evaluated at PBEh(). We find that is
also system-dependent and converges with increasing oligomer length, allowing
to capture the dependence of IP and other electronic properties.Comment: 22 pages with 9 figures, submitted to Physical Review
Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations
The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia
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