QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

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QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

The application of variational density functional perturbation theory (DFPT) to lattice dynamics and dielectric properties is discussed within the plane-wave pseudopotential formalism. We derive a method to calculate the linear response of the exchange-correlation potential in the GGA at arbitrary wavevector. We introduce an efficient self-consistent solver based on all-bands conjugate-gradient minimization of the second order energy, and compare the performance of preconditioning schemes. Lattice-dynamical and electronic structure consequences of space-group symmetry are described, particularly their use in reducing the computational effort required. We discuss the implementation in the CASTEP DFT modeling code, and how DFPT calculations may be efficiently performed on parallel computers. We present results on the lattice dynamics and dielectric properties of $\ensuremath{\alpha}$-quartz, the hydrogen bonded crystal $\mathrm{Na}\mathrm{H}{\mathrm{F}}_{2}$ and the liquid-crystal-forming molecule 5CB. Excellent agreement is found between theory and experiment within the GGA.

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Variational density-functional perturbation theory for dielectrics and lattice dynamics.

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Constrained density functional theory.

Constrained Density Functional Theory

General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg−Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Qx spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S1 → S0 and S0 → S1 of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by ∼10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promo...

Theory of excited state decays and optical spectra: application to polyatomic molecules.

Efficient linear scaling geometry optimization and transition-state search for direct wavefunction optimization schemes in density functional theory using a plane-wave basis

Coordination-dependent interatomic potentials are proposed for silicon oxides and oxynitrides---also hydrogenated ones---with a functional form based on the widely used Tersoff silicon potential. They are intended for an accurate sampling of the configurational space of realistic silicon oxynitride systems and their interfaces with silicon, including defects and changes of oxidation states. The parameters, which are given in the text, are obtained by simultaneously mapping forces and energies onto the results of density-functional-theory calculations performed for a set of diverse systems and configurations and a wide composition range. Application to a larger set of systems and configurations shows the transferability of these augmented Tersoff potentials and their validity in predicting bulk lattice parameters, energetics of defect relaxation, and vibrational spectra.

Ab initio derived augmented Tersoff potential for silicon oxynitride compounds and their interfaces with silicon

By combining ab initio calculations and classical molecular dynamics, we determine how the inclusion of nitrogen in a silica matrix changes its dielectric constant, and elucidate the underlying mechanisms. We find that there is an entire range of nitrogen concentrations (up to approximately 25%) for which the structural pattern of the oxide is preserved in bulk SiON, and the dielectric constant increases mainly because of the variation of the ionic polarizability. This behavior is not sensitive to hydrogen passivation of nitrogen. The few defects, which are associated with electron states near the gap, are mainly centered on undercoordinated nitrogen and undercoordinated silicon, and tend to be removed by hydrogen.

Effects of nitridation on the characteristics of silicon dioxide: dielectric and structural properties from ab initio calculations.

The structure of the interface between SiO2 and Si(100) is investigated using the replica-exchange method driven by classical molecular dynamics simulations based on ab initio-derived interatomic potentials. Abrupt interfaces are shown to be unstable, whereas a substoichiometric oxide forms at the transition between the two materials that exhibits Si atoms in all three intermediate oxidation states, in agreement with experiment. A number of physical characteristics are found to be consistent with experimental data, including the distribution of Si atoms with different oxidation states, the increase in atom density and the stability of a pseudo-cristobalite pattern at the interface as well as the presence of Si–O–Si bridge bonds between the substrate and the suboxide.

The structure of the SiO2∕Si(100) interface from a restraint-free search using computer simulations

This paper proposes methods for calculating the derivative couplings between adiabatic states in density-functional theory (DFT) and compares them with each other and with multiconfigurational self-consistent field calculations. They are shown to be accurate and, as expected, the costs of their calculation scale more favorably with system size than post-Hartree-Fock calculations. The proposed methods are based on single-particle excitations and the associated Slater transition-state densities to overcome the problem of the unavailability of multielectron states in DFT which precludes a straightforward calculation of the matrix elements of the nuclear gradient operator. An iterative scheme employing linear-response theory was found to offer the best trade-off between accuracy and efficiency. The algorithms presented here have been implemented for doublet-doublet excitations within a plane-wave-basis and pseudopotential framework but are easily generalizable to other excitations and basis sets. Owing to their fundamental importance in cases where the Born-Oppenheimer separation of motions is not valid, these derivative couplings can facilitate, for example, the treatment of nonadiabatic charge transfers, of electron-phonon couplings, and of radiationless electronic transitions in DFT.

Salomon R. Billeter

Papers

Efficient linear scaling geometry optimization and transition-state search for direct wavefunction optimization schemes in density functional theory using a plane-wave basis

Ab initio derived augmented Tersoff potential for silicon oxynitride compounds and their interfaces with silicon

Effects of nitridation on the characteristics of silicon dioxide: dielectric and structural properties from ab initio calculations.

The structure of the SiO2∕Si(100) interface from a restraint-free search using computer simulations

Calculation of nonadiabatic couplings in density-functional theory.