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.

Electron correlation effects on the hydrogen passivation of Mn x Ga 1-x As dilute magnetic semiconductors

Abstract: Atomic hydrogen diffuses in semiconductor lattices and binds to impurities by forming complexes that can lead to a full neutralization of the impurity effects. In the present paper, the structural, vibrational, electronic, and magnetic properties of complexes formed by H in the ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ $(x=0.03)$ dilute magnetic semiconductor have been investigated by using first-principles density-functional theory theoretical methods both in gradient-corrected spin-density ($\ensuremath{\sigma}$-GGA) and Hubbard $U(\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{GGA}+U)$ approximations. The results account for recent experimental findings showing a H passivation of the electronic and magnetic properties of Mn in GaAs. Most importantly, they show that electron correlation has crucial effects on the properties of H-Mn complexes.

Structure, electronic properties, and formation mechanisms of hydrogen-nitrogen complexes in GaP y N 1-y alloys

Abstract: The structure, formation energies, chemical bonding, and electronic properties of N-H complexes in the GaP 0 . 9 7 N 0 . 0 3 alloy have been investigated by density functional theory and local density approximation theoretical methods. The achieved results closely parallel those previously found in the case of the GaAs 0 . 9 7 N 0 . 0 3 alloy. In particular, they show that a same H* 2 -like complex can neutralize the N effects on the GaP band structure as it does in the case of GaAsN. These results can account for the H passivation of the N effects observed in GaPN; however, they do not explain some differences in the optical behavior of hydrogenated GaPN and GaAsN. We have investigated therefore the formation mechanism of the H* 2 -like complex. The resulting model suggests that H + ions diffusing in the GaP (GaAs) lattice form an intermediate dihydrogen complex which then transforms into the H* 2 -like one. Thus, the existence of the complex responsible of the N passivation is related to the presence and motion of H + in the GaP and GaAs lattices. In this concern, we have estimated that the bonds formed by H + in GaP are stronger than those formed in GaAs, thus inducing a slower motion of these ions in GaP. This can result in different efficiencies of the hydrogenation procedure in GaPN and GaAsN, which may account for the different behavior of the two alloys upon hydrogenation.

Notes on pseudopotential generation

Abstract: When I started to do my first first-principle calculation (that is, my first-principle calculation) with Stefano Baroni on CsI under pressure (1985), it became quickly evident that the available pseudopotentials (PP’s) couldn’t do the job. So we generated our own PP’s. Since that first experience I have generated a large number of PP’s and people keep asking me new PP’s from time to time. I am happy that ”my” PP’s are appreciated and used by other people. However I don’t think that the generation of PP’s is such a hard task that it requires an official (or unofficial) PP wizard to do this. For this reason I want to share here my (little) experience and the (primitive) computer codes I am using.

Vibrational frequencies of Si-P-H complexes in crystalline silicon: A theoretical study.

Abstract: Les energies du mode H de courbure et d'etirement sont estimees pour les sites stables et metastables de l'atome H. Leurs valeurs pour le site stable concordent avec celles des 2 raies d'absorption infra-rouge

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.