Paolo Giannozzi

Bio: Paolo Giannozzi is an academic researcher from University of Udine. The author has contributed to research in topics: Density functional theory & Ab initio. The author has an hindex of 38, co-authored 122 publications receiving 44408 citations. Previous affiliations of Paolo Giannozzi include Nest Labs & École Polytechnique Fédérale de Lausanne.

Structure and Thermodynamics of SiGe Alloys from Computational Alchemy

Abstract: We present a new method for studying theoretically the structural properties of semiconductor alloys. The alloy is considered as a perturbation with respect to a periodic virtual crystal, and the relevant energies calculated by density-functional perturbation theory. We show that —up to second order in the perturbation— the energy of the alloy is equivalent to that of a lattice gas with only two-body interactions. The interaction constants of the lattice gas are particular linear response functions of the virtual crystal, which can be determined from first principles. Once the interaction constants have been calculated, the finite-temperature properties of the alloy can be studied rather inexpensively by MonteCarlo simulations on the lattice gas. As an application, we consider the case of SixGe1-x. A comparison with traditional self-consistent calculations for some simple ordered structures demonstrates that the accuracy of the perturbative approach is in this case of the same order as that of state-of-the-art density-functional calculations. Ignoring lattice relaxation, the range of the interactions is very short. Atomic relaxation renormalizes the interactions and makes them rather long range, propagating mainly along the bond chains. Monte Carlo simulations show that SixGe1-x is a model random alloy with a miscibility gap below≈ 170 K. The bond length distribution displays three well defined peaks whose positions depend on composition, but not on temperature. The resulting lattice parameter follows very closely Vegard’s law.

Defect engineering in III–V ternary alloys: Effects of strain and local charge on the formation of native deep defects

Abstract: The effects of external and internal strains, and of defect charges on the formation of vacancies and antisites in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that a proper use of strain and defect charges permits the development of a defect engineering of III–V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while the formation of vacancies may be favored only by extreme conditions of compressive strain.

Ab-initio molecular dynamics studies of microclljsters

Abstract: The study of the structural and electronic properties of microclusters is a field of growing interest. Ab-initio molecular dynamics has provided a new and important tool for the theoretical approach to these questions. Here we present some very recent results on small semiconductor aggregates with special reference to calculations of equilibrium shapes and temperature effects. Results of simulations on alkali-metal microclusters are briefly mentioned.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

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

Abstract: 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.

First-principles simulation: ideas, illustrations and the CASTEP code

Abstract: First-principles simulation, meaning density-functional theory calculations with plane waves and pseudopotentials, has become a prized technique in condensed-matter theory. Here I look at the basics of the suject, give a brief review of the theory, examining the strengths and weaknesses of its implementation, and illustrating some of the ways simulators approach problems through a small case study. I also discuss why and how modern software design methods have been used in writing a completely new modular version of the CASTEP code.

Phonons and related crystal properties from density-functional perturbation theory

Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.