Computational Materials Design w/ 1st Principles Quantum Mechanics

atomistic architecture of a material is key to material sciences. first principle quantum mechanical calculations allows capability to predict atomistic structures. investigation of site-selective adsorption of carbon atoms on A1 surfaces. Carbon is a common contaminant of aluminum surfaces, carbon is adsorbed on Al surface in 2 geometries (1) a fourfold-coordinated site (2) a sixfold-coordinated site. more stable by .1ev / atom Based on first-principles density functional theory with a local density approximation and a pseudopotential plane wave method. "Density functional" relates the total electron density and the total energy. Thus derive effective 1-particle Schrödinger equations, which solve iteratively to give the electronic states, the charge density and the total energy. S +

Off-the Shelf chips conquer the Heights of computing

Supercomputer industry is taking a turn. Through history, supercomputers have been built w/ specialized processors. pricey custom-made computer chips for unmatched performance . But massproduced microprocessors used in computer workshtations are fast enough now, that silicon graphics , IBM, et al. are building state-of-the-art supercomputers by stringing together less powerful but far less expensive components. Commodity microprocessors continue to improve far faster than the custom-built couterparts. Applications reflect what problems are amenable to the particular type of supercomputer. Since the new generation can deal w/ larger databases, "datamining" looking for relationships in large data sets will grow in popularity In the 80's battle of the vector supercomputers. Cray was the most powerful because it used custom processors to perform calculations simultaneously on long strings of numbers - vectors - instead of +,-,*,/ two at a time. Instead of relying on a few high power, high cost processors, why not amass hundreds of thousands of cheaper, lowpower processors and provide the same computing power at a fraction of the cost? MPP Born! (Massively parallel processing). powerful, Commodity microprocessors changed everything. Mass-produced chips at the heart of computer workstations. 1985 : Workstation microprocessors 1megaflop Cray's Processors 200megaflops. (1 flop = 15digit#*15digit#) 1995 : Cray Processor = 2gigaflobs Microprocessors DEC Alpha 600megaflop Microprocessors HP's 800megaflop (12/95) Microprocessor cost is a fraction of traditional vector processor. Vector processors, consist of complex collections of chips, the microprocessor are designed for mass production. Each is a self-contained miniature computer made on a single bit of silicon only a centimeter or so across, in a process honed by 2 decades of experience making integrated circuits. The inexpensive power of the microprocessor has made possible a new type of parallel supercomputer. Known as the symmetric multiprocesser (SMP) , it connects up to 220 commodity microprocessors to a single large memory instead of distributing the memory among its processors as MPPs do. 350 |____ n | \____ _______massively parallel u | _\_/_ m | ___/ \____parallel vector b 150 |___/ e | symmetric multiprocessor (microprocessor based) r | ___/ | ___/ 0 +--------------------- Silicon graphics, 18microprocessor, each 300+megaflops peak speeds : >5gigaflops. cost : $1 million Cray 16procC90(flagship), peak speed : 16gigflops cost : $30 million The lower price tag has made SMPs *very* popular. It's possible to make an SMP cluster. Several machines connected with fiber optic cable coordinated through S/W. MPPs are coming to dominate the top end of the market thanks to commodity microprocessors - which has more memory than traditional supercomputers since each processor has its own memory. The extra memory suits the machines to data-intensive applications. Imaging or comparing observational data w/ predictions of models. 100 | \ MIPS R2000 | \ microprocessors n | \ HP 7000 a | ___ \ MIPS R4000 n | \___ \ s 10| \___ \ e | \___\ DEC Alpha c | vector 1| supercomputers | Cray 1S, XMP,YMP,C90 +--------------------- 75 1995 Chips are up. Microprocessors now rival vector processors in speed and number of top supercomputers.
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Computer Scientists rethink their discipline's foundations

Quantum Dot Computers : Possible Speed Increase : 100-10000 depending on size of the smallest dots that can be produced in regular arrays. Current Status : Experimental tests are just beginning (1995) Negatives : Early systems will operate at low temperatures How : replace transistors with spots of semiconductor so small they accomodate 1 electron each. At small scales, electrons behave not like point particles, but as waves. So, hard to handle when circuit elements @ those scales. As e- move wave function spread out, making them apt to tunnel through thin walls between circuit elements and cause circuit to malfunction. Trap individual e- wi/ quantum dots, blobs of semiconductor so tiny an e- around an atomic nucleus the e- wave in a dot is forced to occupy a specific energy state. Narrowing the wave and holding it tight. If laid out at corners of a square, electron's mutual repulsion will force them to burrow to opposite corners of pattern. o---X X---o \ / \ / o "1" or o "0" / \ / \ X---o o---X Quantum Computers : Possible Speed Increase : A trillion or more Current Status : Experiments to create single logic gates are under way. Negatives : Limited range of problems that such computers can solve; exponential accumulation of errors. How : Build components of quantum computers take advantage of single electron wave's ability to exist in many different state at once. Eachstate representing a piece of information. All processed in parallel. Holographic Association Possible Speed Increase : <=100,000 Current Status : Experiments in a variety of holographic media Negatives : Limited to specific applications e.g. pattern recognition, AI How : use a single large laser, bright enough to force a strong response from existing nonlinear materials to drive many computations at once. encode light beams with images or digital information and mix them in nonlinear media as diverse as gases , crystals, and bacterial proteins. The nonlinear medium allows information in one light beam to affect how the material "processes" a second beam. In effect, the medium performs many computations in parallel. Enabling "holographic association" to compare two data sets tens of thousands of times faster than existing supercomputers. Optical Computers : Possible Speed Increase : 1000-100000 wo/ accounting for extra parallelism since photons can pass through one another Current Status : Individual components exist Negatives : Better switches need to be developed How : Light pulses fastest messenger in nature, pass through one another without effect. Allows any number of activities to take place simultaneously in an optical circuit. Some optics researchers aare learning how to make light guides to carry photons around on a chip. Tiny, superefficient lasers generate pulses of light at play in an optical circuit. Induced lasing in a ring of semiconductor 4.5microns across. Ring= resonating cavity intensifying laser light. Since circumference is only several wavelengths long, other wavelengths can't resonate within it and siphon off energy. The laser photons escape from the ring to a surrounding waveguide, where they drive an optical circuit. problen:Such a circuit switches 1 light signal to alter another (qv transistor) DNA Computers : Possible Speed Increase : <=1,000,000,000,000 Current Status : Proof or principle and more sophisticated tests begin Negatives : Limited range of problems, errors, potential practical barriers How : simultaneous parallelism via soup of DNA molecules. 2^70 DNA molecules (a few liters) act as individual processors. Nucleotide sequence of each molecule encodes a possible solution to the problem. By applying the techniques of molecular biology to clone,combine and select subsets of the molecules, the operator of this biochemical computer can force the system to sort through the entire astronomical range of possible solutions, leaving the correct sequence to be extracted and read out. Each step takes minutes to hours, vs billionth of a second / operation for a supercomputer - but each step also acts on the whole panoply of molecules at once. The result in the most optimisic scenarios, is a theoretical advantage of 10^8 - 10^12 in computing speed. The advantage holds only for problems solved efficienty by following many parallel computational paths (e.g. cryptography). Applications : model global climate, embryonic development elementary particle interactions, simultaneously optimizing jet's structural mechanics acoustics, manuf & $ S +

Keplerian complexity:Numerical simulations of accretion disk transport

Supercomputer simulations have been used in conjunction with analytic studies to investigate the central issue of astrophysical accretion-disk dynamics: the nature of the angular momentum transport. Simulations provide the means to investigate and experiment with candidate mechanisms , including global hydrodynamic instabilities, spiral shock waves, and local magnetohydrodynamic instabilities. Simulations have demonstrated that accretion disks are generally MHD turbulent. These results suggest that the fundamental physical mechanism for angular momentum transport in accretion disks has now been identified. --fin