Roberto Car

Bio: Roberto Car is an academic researcher from Princeton University. The author has contributed to research in topics: Density functional theory & Ab initio. The author has an hindex of 99, co-authored 389 publications receiving 76681 citations. Previous affiliations of Roberto Car include International School for Advanced Studies & University of Geneva.

Longitudinal polarizability of long polymeric chains: quasi-one-dimensional electrostatics as the origin of slow convergence.

Abstract: The longitudinal linear polarizability alpha(N) of a stereoregular oligomer of size N is proportional to N in the large-N limit, provided the system is nonconducting in that limit. It has long been known that the convergence of alpha(N)/N to the asymptotic alpha(infinity) value is slow. We show that the leading term in the difference between alpha(N)/N and alpha(infinity) is of the order of 1/N. The difference [alpha(N)-alpha(N-1)], as well as alpha(center)(N) (when computationally accessible), also converge to alpha(infinity), but faster, the leading term being of the order of 1/N(2). We also present evidence that in these cases the power law convergence behavior is due to quasi-one-dimensional electrostatics, with one exception. Specifically, in molecular systems the difference between alpha(N)/N and alpha(infinity) has not just one but two sources of the O(1/N) term, with one being due to the aforementioned Coulomb interactions, and the second due to the short ranged exponentially decaying perturbations on chain ends. The major role of electrostatics in the convergence of the remainders is demonstrated by means of a Clausius-Mossotti-type classical model. The conclusions derived from the model are also shown to be applicable in molecular systems, by means of test-case ab initio calculations on linear stacks of H(2) molecules, and on polyacetylene chains. The implications of the modern theory of polarization for extended systems are also discussed.

Quasi-one-dimensional K-O chain in PTCDA thin films: evidence from first-principles calculations.

Abstract: Using density functional theory calculations we have found that K atoms in a PTCDA ($3,4\ensuremath{\mathbin:}9,10$-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic oxygen of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 \AA{}) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PTCDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eV in the K-doped system, confirming that the interaction occurs between different molecular planes.

Theoretical study of SiH 3 -defect stability and formation on the H-saturated Si(100)1×1 surface

Abstract: We present first-principles calculations for the initial stages of etching of Si(100) by H atoms We find that adsorbed ${\mathrm{SiH}}_{3}$ species [(${\mathrm{SiH}}_{3}$(a)] on the H-saturated Si(100)1\ifmmode\times\else\texttimes\fi{}1 surface have a negative formation energy Our results also indicate that the Si-Si backbond breaking which gives rise to the formation of these ${\mathrm{SiH}}_{3}$(a) defects is possibly promoted by a bonding state between hydrogen and silicon atoms in the second layer

Surface phonons and dipole activity of Si(111)2×1 from ab initio calculations

Abstract: Clarification d'une controverse sur l'attribution du pic de perte d'energie des electrons a environ 56 meV. La contribution dominante au pic de perte est anisotrope et provient d'une vibration optique longitudinale des chaines de surface. Cependant une faible contribution, mais non negligeable, provient de modes des soussurfaces polarises, perpendiculaires a la surface

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.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.