35 research outputs found
Self-consistent DFT+U method for real-space time-dependent density functional theory calculations
We implemented various DFT+U schemes, including the ACBN0 self-consistent
density-functional version of the DFT+U method [Phys. Rev. X 5, 011006 (2015)]
within the massively parallel real-space time-dependent density functional
theory (TDDFT) code Octopus. We further extended the method to the case of the
calculation of response functions with real-time TDDFT+U and to the description
of non-collinear spin systems. The implementation is tested by investigating
the ground-state and optical properties of various transition metal oxides,
bulk topological insulators, and molecules. Our results are found to be in good
agreement with previously published results for both the electronic band
structure and structural properties. The self consistent calculated values of U
and J are also in good agreement with the values commonly used in the
literature. We found that the time-dependent extension of the self-consistent
DFT+U method yields improved optical properties when compared to the empirical
TDDFT+U scheme. This work thus opens a different theoretical framework to
address the non equilibrium properties of correlated systems
Generating relativistic pseudo-potentials with explicit incorporation of semi-core states using APE, the Atomic Pseudo-potentials Engine
We present a computer package designed to generate and test norm-conserving pseudo-potentials within Density Functional Theory. The generated pseudo-potentials can be either non-relativistic, scalar relativistic or fully relativistic and can explicitly include semi-core states. A wide range of exchange-correlation functionals is included.Program summaryhttp://www.sciencedirect.com/science/article/B6TJ5-4R7J89B-3/1/539d407a3f8f5790a3d31b2c8baa5ff
On the use of Neumann's principle for the calculation of the polarizability tensor of nanostructures
The polarizability measures how the system responds to an applied electrical
field. Computationally, there are many different ways to evaluate this
tensorial quantity, some of which rely on the explicit use of the external
perturbation and require several individual calculations to obtain the full
tensor. In this work, we present some considerations about symmetry that allow
us to take full advantage of Neumann's principle and decrease the number of
calculations required by these methods. We illustrate the approach with two
examples, the use of the symmetries in real space and in spin space in the
calculation of the electrical or the spin response.Comment: 7 pages, 5 figures, accepted for publication in the Journal of
Nanoscience and Nanotechnolog
Density-based mixing parameter for hybrid functionals
A very popular ab-initio scheme to calculate electronic properties in solids
is the use of hybrid functionals in density functional theory (DFT) that mixes
a portion of Fock exchange with DFT functionals. In spite of their success, a
major problem still remains, related to the use of one single mixing parameter
for all materials. Guided by physical arguments that connect the mixing
parameter to the dielectric properties of the solid, and ultimately to its band
gap, we propose a method to calculate this parameter from the electronic
density alone. This method is able to cut significantly the error of
traditional hybrid functionals for large and small gap materials, while
retaining a good description of structural properties. Moreover, its
implementation is simple and leads to a negligible increase of the
computational time.Comment: submitte
Band widths and gaps from the Tran-Blaha functional : Comparison with many-body perturbation theory
For a set of ten crystalline materials (oxides and semiconductors), we
compute the electronic band structures using the Tran-Blaha [Phys. Rev. Lett.
102, 226401 (2009)] (TB09) functional. The band widths and gaps are compared
with those from the local-density approximation (LDA) functional, many-body
perturbation theory (MBPT), and experiments. At the density-functional theory
(DFT) level, TB09 leads to band gaps in much better agreement with experiments
than LDA. However, we observe that it globally underestimates, often strongly,
the valence (and conduction) band widths (more than LDA). MBPT corrections are
calculated starting from both LDA and TB09 eigenenergies and wavefunctions.
They lead to a much better agreement with experimental data for band widths.
The band gaps obtained starting from TB09 are close to those from
quasi-particle self-consistent GW calculations, at a much reduced cost.
Finally, we explore the possibility to tune one of the semi-empirical
parameters of the TB09 functional in order to obtain simultaneously better band
gaps and widths. We find that these requirements are conflicting.Comment: 18 pages, 16 figure
Accuracy of generalized gradient approximation functionals for density functional perturbation theory calculations
We assess the validity of various exchange-correlation functionals for
computing the structural, vibrational, dielectric, and thermodynamical
properties of materials in the framework of density-functional perturbation
theory (DFPT). We consider five generalized-gradient approximation (GGA)
functionals (PBE, PBEsol, WC, AM05, and HTBS) as well as the local density
approximation (LDA) functional. We investigate a wide variety of materials
including a semiconductor (silicon), a metal (copper), and various insulators
(SiO -quartz and stishovite, ZrSiO zircon, and MgO periclase).
For the structural properties, we find that PBEsol and WC are the closest to
the experiments and AM05 performs only slightly worse. All three functionals
actually improve over LDA and PBE in contrast with HTBS, which is shown to fail
dramatically for -quartz. For the vibrational and thermodynamical
properties, LDA performs surprisingly very good. In the majority of the test
cases, it outperforms PBE significantly and also the WC, PBEsol and AM05
functionals though by a smaller margin (and to the detriment of structural
parameters). On the other hand, HTBS performs also poorly for vibrational
quantities. For the dielectric properties, none of the functionals can be put
forward. They all (i) fail to reproduce the electronic dielectric constant due
to the well-known band gap problem and (ii) tend to overestimate the oscillator
strengths (and hence the static dielectric constant)
Computational benchmarking for ultrafast electron dynamics: wavefunction methods vs density functional theory
Attosecond electron dynamics in small- and medium-sized molecules, induced by an ultrashort strong optical pulse, is studied computationally for a frozen nuclear geometry. The importance of exchange and correlation effects on the nonequilibrium electron dynamics induced by the interaction of the molecule with the strong optical pulse is analyzed by comparing the solution of the time-dependent Schrodinger equation based on the correlated field-free stationary electronic states computed with the equation-of-motion coupled cluster singles and doubles and the complete active space multi-configurational self-consistent field methodologies on one hand, and various functionals in real-time time-dependent density functional theory (TD-DFT) on the other. We aim to evaluate the performance of the latter approach, which is very widely used for nonlinear absorption processes and whose computational cost has a more favorable scaling with the system size. We focus on LiH as a toy model for a nontrivial molecule and show that our conclusions carry over to larger molecules, exemplified by ABCU (C10H19N). The molecules are probed with IR and UV pulses whose intensities are not strong enough to significantly ionize the system. By comparing the evolution of the time-dependent field-free electronic dipole moment, as well as its Fourier power spectrum, we show that TD-DFT performs qualitatively well in most cases. Contrary to previous studies, we find almost no changes in the TD-DFT excitation energies when excited states are populated. Transitions between states of different symmetries are induced using pulses polarized in different directions. We observe that the performance of TD-DFT does not depend on the symmetry of the states involved in the transition.Belgian Fonds National de la Recherche Collective through project number 2.4545.12 “Control of attosecond dynamics: applications to molecular reactivity
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Time-Dependent Density-Functional Theory in Massively Parallel Computer Architectures: The Octopus Project
Octopus is a general-purpose density-functional theory (DFT) code, with a particular emphasis on the time-dependent version of DFT (TDDFT). In this paper we present the ongoing efforts to achieve the parallelization of octopus. We focus on the real-time variant of TDDFT, where the time-dependent Kohn–Sham equations are directly propagated in time. This approach has great potential for execution in massively parallel systems such as modern supercomputers with thousands of processors and graphics processing units (GPUs). For harvesting the potential of conventional supercomputers, the main strategy is a multi-level parallelization scheme that combines the inherent scalability of real-time TDDFT with a real-space grid domain-partitioning approach. A scalable Poisson solver is critical for the efficiency of this scheme. For GPUs, we show how using blocks of Kohn–Sham states provides the required level of data parallelism and that this strategy is also applicable for code optimization on standard processors. Our results show that real-time TDDFT, as implemented in octopus, can be the method of choice for studying the excited states of large molecular systems in modern parallel architectures.Chemistry and Chemical Biolog