40 research outputs found
Optimization Algorithm for the Generation of ONCV Pseudopotentials
We present an optimization algorithm to construct pseudopotentials and use it
to generate a set of Optimized Norm-Conserving Vanderbilt (ONCV)
pseudopotentials for elements up to Z=83 (Bi) (excluding Lanthanides). We
introduce a quality function that assesses the agreement of a pseudopotential
calculation with all-electron FLAPW results, and the necessary plane-wave
energy cutoff. This quality function allows us to use a Nelder-Mead
optimization algorithm on a training set of materials to optimize the input
parameters of the pseudopotential construction for most of the periodic table.
We control the accuracy of the resulting pseudopotentials on a test set of
materials independent of the training set. We find that the automatically
constructed pseudopotentials provide a good agreement with the all-electron
results obtained using the FLEUR code with a plane-wave energy cutoff of
approximately 60 Ry.Comment: 11 pages, 6 figure
A Finite-field Approach for Calculations Beyond the Random Phase Approximation
We describe a finite-field approach to compute density response functions,
which allows for efficient and calculations beyond
the random phase approximation. The method is easily applicable to density
functional calculations performed with hybrid functionals. We present results
for the electronic properties of molecules and solids and we discuss a general
scheme to overcome slow convergence of quasiparticle energies obtained from
calculations, as a function of the basis set used to represent
the dielectric matrix
Ab initio investigation of the melting line of nitrogen at high pressure
Understanding the behavior of molecular systems under pressure is a
fundamental problem in condensed matter physics. In the case of nitrogen, the
determination of the phase diagram and in particular of the melting line, are
largely open problems. Two independent experiments have reported the presence
of a maximum in the nitrogen melting curve, below 90 GPa, however the position
and the interpretation of the origin of such maximum differ. By means of ab
initio molecular dynamics simulations based on density functional theory and
thermodynamic integration techniques, we have determined the phase diagram of
nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting
line, related to a transformation in the liquid, from molecular N_2 to
polymeric nitrogen accompanied by an insulator-to-metal transition
A finite field approach to solving the Bethe Salpeter equation
We present a method to compute optical spectra and exciton binding energies
of molecules and solids based on the solution of the Bethe-Salpeter equation
(BSE) and the calculation of the screened Coulomb interaction in finite field.
The method does not require the explicit evaluation of dielectric matrices nor
of virtual electronic states, and can be easily applied without resorting to
the random phase approximation. In addition it utilizes localized orbitals
obtained from Bloch states using bisection techniques, thus greatly reducing
the complexity of the calculation and enabling the efficient use of hybrid
functionals to obtain single particle wavefunctions. We report exciton binding
energies of several molecules and absorption spectra of condensed systems of
unprecedented size, including water and ice samples with hundreds of atoms
Engineering the formation of spin-defects from first principles
The full realization of spin qubits for quantum technologies relies on the
ability to control and design the formation processes of spin defects in
semiconductors and insulators. We present a computational protocol to
investigate the synthesis of point-defects at the atomistic level, and we apply
it to the study of a promising spin-qubit in silicon carbide, the divacancy
(VV). Our strategy combines electronic structure calculations based on density
functional theory and enhanced sampling techniques coupled with first
principles molecular dynamics. We predict the optimal annealing temperatures
for the formation of VVs at high temperature and show how to engineer the Fermi
level of the material to optimize the defect's yield for several polytypes of
silicon carbide. Our results are in excellent agreement with available
experimental data and provide novel atomistic insights into point defect
formation and annihilation processes as a function of temperature
Roadmap on semiconductor-cell biointerfaces.
This roadmap outlines the role semiconductor-based materials play in understanding the complex biophysical dynamics at multiple length scales, as well as the design and implementation of next-generation electronic, optoelectronic, and mechanical devices for biointerfaces. The roadmap emphasizes the advantages of semiconductor building blocks in interfacing, monitoring, and manipulating the activity of biological components, and discusses the possibility of using active semiconductor-cell interfaces for discovering new signaling processes in the biological world